* feat!: Adding Pluggable Auth Support to ADC (#1419)

* feat: Adding Pluggable Auth Support to ADC

See go/pluggable-auth-design.
Adding classes required for supporting pluggable auth and some functionality.
Will add the implementation for running the executable and reading from a cached file in a later pull request.

* fix: Correcting copyright year

* fix: addressing code review comments

* fix: Fixing interface description

* fix: Comment typo

* fix: Address code review comments

* fix: Add comments to ExecutableResponse properties

* fix: Addressing code review comments

* feat: adding executable and file handling for Pluggable Auth Client (#1431)

* feat: adding executable and file handling for pluggable auth client

Added PluggableAuthHandler to run user provided executable and read from cached file output + associated tests.

* fix: correcting pluggable auth credential source

Adding 'executable' object under credential source for pluggable auth options.

* fix: code review comments

* fix: Fixing output file string variable name and failing tests

* fix: code review comments + added readme documentation.

* fix: addressing code review

* 🦉 Updates from OwlBot post-processor

See https://github.com/googleapis/repo-automation-bots/blob/main/packages/owl-bot/README.md

* fix: addressing code review

* fix: fixing test

* fix: fixing linter warning

* fix: Adding back in readme changes that got removed by owlbot

* 🦉 Updates from OwlBot post-processor

See https://github.com/googleapis/repo-automation-bots/blob/main/packages/owl-bot/README.md

Co-authored-by: Owl Bot <gcf-owl-bot[bot]@users.noreply.github.com>
Co-authored-by: Leo <[email protected]>

Co-authored-by: release-please[bot] <55107282+release-please[bot]@users.noreply.github.com>