Support Airflow-specific annotations to schedules, including environment-specific schedules

[aaronsteers]

The new annotations spec will be released to Meltano soon. This spec allows projects to define deploy hints and other rules for 3rd-party tools.

schedules:
- name: slack_notifications
  interval: 0 12 * * *
  job: slack_notifications
  annotations:
    airflow:
      disabled: True

- name: cloudwatch_el
  interval: 0 6 * * *
  job: cloudwatch_el
  annotations:
    airflow:
      environments: [prod, staging]

- name: meltanohub_el_staging
  interval: 0 6 * * *
  job: meltanohub_el
  annotations:
    airflow:
      environments: [prod]

- name: spreadsheets_anywhere_el
  interval: 0 6 * * *
  job: spreadsheets_anywhere_el
  annotations:
    airflow:
      environments: [prod]

- name: daily_staging_dry_run
  interval: 0 6 * * *
  job: spreadsheets_anywhere_el
  annotations:
    airflow:
      environments: [staging]

The logic here will be ignored by Meltano Core - but it can be used within airflow-ext to modify how schedules will be executed.

The above logic handles two basic rules:

1. If disabled=True, then the job would be disabled for all environments - or simply not deployed to Airflow at all.
2. If environments is declared, the schedule will only be run if it contains a match to the name of the currently activated Meltano Environment.

[aaronsteers]

WIP PR:

• feat: apply annotations to determine if schedules should be enabled #36