dest: Destination display error handling

[mastercactapus]

Description:
Fixes error handling with the new destination API. Namely, it prevents the entire page from breaking (e.g., the escalation policy details page) if any single action is invalid (e.g., archived or invalid slack channel). Instead individual chips/line items will render an error.

The purpose is to ensure in such an event, a user is able to interact with the page and take action to resolve the issue.

• gql typescript generator tool was updated to support GraphQL unions

Screenshots:

Escalation Policy Details

TODAY

This PR

User Contact Methods

TODAY

This PR

Schedule On-Call Notifications

TODAY

This PR

Describe any introduced user-facing changes:
N/A fixing broken behavior

Describe any introduced API changes:
!! Breaking changes to the destination's API !!

• FieldValuePair no longer contains label and isFavorite these were moved to FieldSearchResult to separate the two
• Destination.displayInfo return type changed from DestinationDisplayInfo to a union of DestinationDisplayInfo and DestinationDisplayInfoError -- rather than failing the entire query with a global error, errors are now returned inline
• destinationFieldSearch return type changed to FieldSearchConnection, which was renamed from FieldValueConnection to match FieldSearchResult

Additional Info:
The affected APIs are only used when the dest-types experimental flag is enabled.

[ethan-haynes]

Doesn't have to be a part of this PR, but maybe we should write some additional tests to catch this type of behavior from happening with other areas we are making changes in.