Add Signing To Commit Function

[hi-artem]

Add gpg signing

Fix #274

Add ability to specify gpg key and passphrase to sign commits. No passphrase will be treated as empty passphrase.

Test

Tested manually with this config for scm:

scms:
  default:
    kind: github
    spec:
      user: "{{ .github.user }}"
      email: "{{ .github.email }}"
      owner: "{{ .github.owner }}"
      repository: "{{ .github.repository }}"
      token: "{{ requiredEnv .github.token }}"
      username: "{{ .github.username }}"
      branch: "{{ .github.branch }}"
      gpg:
        signingKey: |
          -----BEGIN PGP PRIVATE KEY BLOCK-----

          Blablabla... this is secret
          -----END PGP PRIVATE KEY BLOCK-----
        enabled: true

Also, added a new test for sign package:

cd updatecli/pkg/plugins/scms/git/sign
go test

Additional Information

Tradeoff

Potential improvement

Any feedback is welcome. I am not familiar with the tool project structure, so if there is a need to change anything in the PR it is cool with me. I am motivated to get it merged.

[olblak]

I wanted to work on this for a quite long time, thanks very much.

[olblak]

The pullrquest looks very good, I'll need some time tomorrow to do additionnal testing

[hi-artem]

@olblak Thank you for reviewing my PR! Looking forward to the testing results.

[olblak]

I confirmed that I successfully manage to sign commits \o/