feat: add Google Cloud Build CI support

[escarls]

Add Google Cloud Build CI detection and build meta-data extraction from the environment variables when they are mapped.

Google Cloud Build uses variable substitutions in the builds and not environment variables.

It is possible to map substitution variables to environment variables in the builds, but it has to be done manually.

For this CI adapter to work, these variables needs to be mapped:

env:
- '_PR_NUMBER=$_PR_NUMBER'
- 'BRANCH_NAME=$BRANCH_NAME'
- 'BUILD_ID=$BUILD_ID'
- 'COMMIT_SHA=$COMMIT_SHA'
- 'LOCATION=$LOCATION'
- 'PROJECT_ID=$PROJECT_ID'
- 'PROJECT_NUMBER=$PROJECT_NUMBER'
- 'REF_NAME=$REF_NAME'
- 'REPO_FULL_NAME=$REPO_FULL_NAME'
- 'TRIGGER_NAME=$TRIGGER_NAME'

[escarls]

I know that the signature is missing, but it seemed to me that it's easier to work through the issues first, then sqash everything and then sign for final review.

[nikolaik]

This looks wonderful, would love to see this land!

[escarls]

Anything missing from my side before a review @joseph-sentry ?