Create Admin API endpoints for managing Aggregators

[joshvote]

The admin API currently only has readonly access to the aggregator , certificate, aggregator_certificate_assignment and aggregator_domain tables.

Implement endpoint(s) with a some upgraded envoy-schema Aggregator models to allow the external management. Primary usecases:

• Create new aggregator
• Create/Remove certificates assigned to an aggregator
• Create/Remove whitelisted domains assigned to an aggregator

[joecrowley-synergy]

Hi @joshvote i'd like to take this one on since Jan (the dev that raised #202) works with me and is looking for it to be implemented.