feat: Sign release packages

[lantoli]

Description

Sign release packages, so CLI can check the signature when the plugin is installed, e.g.:

> plugin atlas plugin install mongodb-labs/atlas-cli-plugin-terraform

PGP signature verification successful!
Plugin mongodb-labs/atlas-cli-plugin-terraform successfully installed

It also signs Windows binaries.

The release has been tested in a fork repo for Linux, Mac and Windows.

Link to any related issue(s): CLOUDP-294109

Type of change:

• Bug fix (non-breaking change which fixes an issue). Please, add the "bug" label to the PR.
• New feature (non-breaking change which adds functionality). Please, add the "enhancement" label to the PR. A migration guide must be created or updated if the new feature will go in a major version.
• Breaking change (fix or feature that would cause existing functionality to not work as expected). Please, add the "breaking change" label to the PR. A migration guide must be created or updated.
• This change requires a documentation update
• Documentation fix/enhancement

Required Checklist:

• I have signed the MongoDB CLA
• I have read the contributing guides
• I have checked that this change does not generate any credentials and that they are NOT accidentally logged anywhere.
• I have added tests that prove my fix is effective or that my feature works per HashiCorp requirements
• I have added any necessary documentation (if appropriate)
• I have run make fmt and formatted my code
• If changes include deprecations or removals I have added appropriate changelog entries.
• If changes include removal or addition of 3rd party GitHub actions, I updated our internal document. Reach out to the APIx Integration slack channel to get access to the internal document.

Further comments

[marcosuma]

Not an expert on signing packages, so good to have another pair of eyes

[cveticm]

LGTM! but would make sure team has no concerns with no signing for MacOS binary before merge

[EspenAlbert]

A bit tricky to follow outside of context. Not sure if it is worth it to include some link/description in the CONTRIBUTING.md?

[lantoli]

@EspenAlbert I've not seen similar info in CLI or Kubernetes plugin, for the user it's only important that they'll see the message:

PGP signature verification successful!
Plugin mongodb-labs/atlas-cli-plugin-terraform successfully installed