Add delete action for gcp bucket

[robertdeheer]

Describe the feature

I'm interested in using cloudcustodian, but have a blocker. I can filter for gcp buckets, but the delete action is not supported. Is that something you would add to your roadmap or should I submit a pull request?

Extra information or context

policies:
  - name: delete-30-day-old-buckets
    description: |
      Delete all buckets greater than 30 days old
    resource: gcp.bucket
    filters:
      - type: value
        key: name
        value: deleteme-0123456
      - type: value
        key: timeCreated
        op: greater-than
        value_type: age
        value: 30
    actions:
      - delete

[ajkerrigan]

A pull request for that would be welcome! Feel free to raise a flag here, in Gitter, or in a community meeting if you have any questions or hit issues along the way. Thanks!