Move golangci-lint options to dedicated config file

[atc0005]

What

Instead of specifying the options like this:

golangci-lint run \
    -E goimports \
    -E gosec \
    -E stylecheck \
    -E goconst \
    -E depguard \
    -E prealloc \
    -E misspell \
    -E maligned \
    -E dupl \
    -E unconvert \
    -E golint \
    -E gocritic

Specify them something like this (copy/paste from another repo, not customized for our linters):

linters:
  enable:
    - dogsled
    - dupl
    - gocognit
    - goconst
    - gocritic
    - gocyclo
    - gofmt
    - golint
    - gosec
    - maligned
    - nakedret
    - prealloc
    - scopelint
    - unconvert
    - unparam
    - whitespace

linters-settings:
  funlen:
    lines: 60
    statements: 40

  gocognit:
    # minimal code complexity to report, 30 by default (but we recommend 10-20)
    min-complexity: 10

  gocyclo:
    # minimal code complexity to report, 30 by default (but we recommend 10-20)
    min-complexity: 15

  golint:
    # minimal confidence for issues, default is 0.8
    min-confidence: 0.3

  maligned:
    # print struct with more effective memory layout or not, false by default
    suggest-new: true

  nakedret:
    # make an issue if func has more lines of code than this setting and it has naked returns; default is 30
    max-func-lines: 2

  unparam:
    # Inspect exported functions, default is false. Set to true if no external program/library imports your code.
    # XXX: if you enable this setting, unparam will report a lot of false-positives in text editors:
    # if it's called for subdir of a project it can't find external interfaces. All text editor integrations
    # with golangci-lint call it on a directory with the changed file.
    check-exported: true

  unused:
    # treat code as a program (not a library) and report unused exported identifiers; default is false.
    # XXX: if you enable this setting, unused will report a lot of false-positives in text editors:
    # if it's called for subdir of a project it can't find funcs usages. All text editor integrations
    # with golangci-lint call it on a directory with the changed file.
    check-exported: false

  whitespace:
    # Enforces newlines (or comments) after every multi-line if statement
    multi-if: true
    # Enforces newlines (or comments) after every multi-line function signature
    multi-func: true

issues:
  # Not using default exclusions because we want to require comments on public
  # functions and types.
  exclude-use-default: false

# options for analysis running
run:
  # include test files or not, default is true
  tests: false

  # by default isn't set. If set we pass it to "go list -mod={option}". From "go help modules":
  # If invoked with -mod=readonly, the go command is disallowed from the implicit
  # automatic updating of go.mod described above. Instead, it fails when any changes
  # to go.mod are needed. This setting is most useful to check that go.mod does
  # not need updates, such as in a continuous integration and testing system.
  # If invoked with -mod=vendor, the go command assumes that the vendor
  # directory holds the correct copies of dependencies and ignores
  # the dependency descriptions in go.mod.
  modules-download-mode: vendor

service:
  # use the fixed version to not introduce new linters unexpectedly
  golangci-lint-version: 1.20.x

Why

By having an external config file (that is picked up automatically), this can be used in the Makefile, in manual golangci-lint runs and (I believe) via Visual Studio Code when golangci-lint is set as the default linter. It would be good to be able to set the linting options in one place and have all applicable use cases covered.

References

• https://github.com/dasrick/go-teams-notify/blob/master/.golangci.yaml
• https://github.com/golangci/golangci-lint#config-file