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/s1ntaxe770r/digger/blob/5815775095d7380281c71c7c3aa63ca1b374365f/pkg/locking/locking.go#L40

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