Add envsubst package

envsubst/README.md

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+# envsubst
+
+`github.com/fluxcd/pkg/envsubst` is a Go package for expanding variables in a string using `${var}` syntax.
+Includes support for bash string replacement functions.
+
+This package is a fork of [drone/envsubst](https://github.com/drone/envsubst).
+
+## Supported Functions
+
+| __Expression__                | __Meaning__                                                         |
+|-------------------------------|---------------------------------------------------------------------|
+| `${var}`                      | Value of `$var`                                                     |
+| `${#var}`                     | String length of `$var`                                             |
+| `${var^}`                     | Uppercase first character of `$var`                                 |
+| `${var^^}`                    | Uppercase all characters in `$var`                                  |
+| `${var,}`                     | Lowercase first character of `$var`                                 |
+| `${var,,}`                    | Lowercase all characters in `$var`                                  |
+| `${var:n}`                    | Offset `$var` `n` characters from start                             |
+| `${var:n:len}`                | Offset `$var` `n` characters with max length of `len`               |
+| `${var#pattern}`              | Strip shortest `pattern` match from start                           |
+| `${var##pattern}`             | Strip longest `pattern` match from start                            |
+| `${var%pattern}`              | Strip shortest `pattern` match from end                             |
+| `${var%%pattern}`             | Strip longest `pattern` match from end                              |
+| `${var-default}`              | If `$var` is not set, evaluate expression as `$default`             |
+| `${var:-default}`             | If `$var` is not set or is empty, evaluate expression as `$default` |
+| `${var=default}`              | If `$var` is not set, evaluate expression as `$default`             |
+| `${var:=default}`             | If `$var` is not set or is empty, evaluate expression as `$default` |
+| `${var/pattern/replacement}`  | Replace as few `pattern` matches as possible with `replacement`     |
+| `${var//pattern/replacement}` | Replace as many `pattern` matches as possible with `replacement`    |
+| `${var/#pattern/replacement}` | Replace `pattern` match with `replacement` from `$var` start        |
+| `${var/%pattern/replacement}` | Replace `pattern` match with `replacement` from `$var` end          |
+
+For a deeper reference, see [bash-hackers](https://wiki.bash-hackers.org/syntax/pe#case_modification) or [gnu pattern matching](https://www.gnu.org/software/bash/manual/html_node/Pattern-Matching.html).
+
+## Unsupported Functions
+
+* `${var+default}`
+* `${var:?default}`
+* `${var:+default}`

envsubst/doc.go

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+/*
+Copyright 2024 The Flux authors
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+// Package envsubst is a Go package for expanding variables in a string using `${var}` syntax.
+// Includes support for bash string replacement functions.
+package envsubst

envsubst/eval.go

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+/*
+Copyright 2024 The Flux authors
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+// Forked from: https://github.com/drone/envsubst
+// MIT License
+// Copyright (c) 2017 drone.io.
+
+package envsubst
+
+import "os"
+
+// Eval replaces ${var} in the string based on the mapping function.
+func Eval(s string, mapping func(string) (string, bool)) (string, error) {
+	t, err := Parse(s)
+	if err != nil {
+		return s, err
+	}
+	return t.Execute(mapping)
+}
+
+// EvalEnv replaces ${var} in the string according to the values of the
+// current environment variables. References to undefined variables are
+// replaced by the empty string.
+func EvalEnv(s string, strict bool) (string, error) {
+	mapping := Getenv
+	if strict {
+		mapping = os.LookupEnv
+	}
+	return Eval(s, mapping)
+}
+
+func Getenv(s string) (string, bool) {
+	return os.Getenv(s), true
+}