PR-level locks

For every pull request we perform a lock when the pull request is opened and unlocked when the pull request is merged, this is to avoid making a plan preview stale

For GCP locking is performed using buckets that are strongly consistent: https://github.com/diggerhq/digger/blob/80289922227f225d887feb74749b4daef8b441f8/pkg/gcp/gcp_lock.go#L13

These options are configured and the locking can be disabled entirely if it is not needed

The locking interface is very simple and is based on Lock() and Unlock() Operations https://github.com/diggerhq/digger/blob/5815775095d7380281c71c7c3aa63ca1b374365f/pkg/locking/locking.go#L40

A pull request acquires a lock for every project impacted by this PR and all dependant projects