Implement API for Goalert

[tommy04062019]

What problem would you like to solve? Please describe: We should have REST API to do some tasks. For example:

• Get service name of a AlertID (we current don't have info about service name in the response returned)
• Get Event log of AlertID(Escalation log...).
• Some CRUD tasks on schedules, rotation, escalation policy...

Describe the solution you'd like: A clear and concise description of what you want to happen.

Describe alternatives you've considered: A clear and concise description of any alternative solutions or features you've considered.

Additional context: Add any other context or screenshots about the feature request here.

[stale[bot]]

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