Support custom branches for CRON deployment triggers in yml specs

It looks like when defining a deployment trigger in cron format, overriding the branch is not supported. Can we add the functionality so when defining a branch in the yml specs, this maps to the current `Use a custom plan branch` option on the trigger configuration.

suggested syntax:

triggers:
  - cron:
      expression: 0 0 * * ? * 
      branch: <branch>