Pulumi ESC support

[NorthIsUp]

Our team uses pulumi for secrets management, so I added it!

We have been using a fork of fnox with this patch for several weeks and it's been working well for us, so here it is for your consideration.

https://github.com/jdx/fnox/compare/main...NorthIsUp:fnox:feat/pulumi-esc-provider?expand=1

the one non-pulumi native addition is a late binding syntax for variables. pulumi normally has the ability to import/reference values from other environments, these imports have an "early binding" syntax using `${variable}` notation. Early binding here means that it immediately tries to interpolate the value with the current available values. I added `%{variable}` syntax that binds late via simple (i.e. non-recursive, non-nested) interpolation inside of fnox. This late binding allows for subclassing + override of developer specific values.

/env/A

values:
  aValue = "A"

envVars:
  some_value = ${aValue}

/env/B

import /env/A

values
  aValue = "B"

in this example /env/B will halve some_value == "A". To get the late binding behavior i added %{} for variables, this is interpolated by fnox before injecting into the shell.

/env/A

values:
  aValue = "A"

envVars:
  some_value = %{aValue}

/env/B

import /env/A

values
  aValue = "B"

In this example some_value == "B" because it uses late binding.