dest: add API support to create EP step with destinations

[tony-tvu]

• Identified the issue which this PR solves.
• Read the CONTRIBUTING document.
• Code builds clean without any errors or warnings.
• Added appropriate tests for any new functionality.
• All new and existing tests passed.
• Added comments in the code, where necessary.
• Ran make check to catch common errors. Fixed any that came up.

Description:
This PR adds support for creating an escalation policy step using generic destinations when the experimental flag is set make start EXPERIMENTAL=dest-types.

Which issue(s) this PR fixes:
part of #2639

Out of Scope:
N/A

Screenshots:
N/A

Describe any introduced user-facing changes:
N/A

Describe any introduced API changes:
Added CreateEscalationPolicyStepInput resolver. Also added favoritesFirst field to DestinationFieldSearchInput schema to show favorites first when searching for rotations, schedules, and users.

Additional Info:
N/A