graphql: add destinationTypes and destinationFieldValidate queries

[mastercactapus]

Description:
Implements two new queries for generic destination support. Additionally, a @experimental directive was added to mark these fields as experimental in the schema.

Run make start EXPERIMENTAL=dest-types to start with this feature turned on.

Query.destinationTypes returns a list of all valid/built-in destination types.

Query.destinationFieldValidate can be used to validate the value of a field.

Examples:

Valid webhook (returns true):

  destinationFieldValidate(input:{
    destType: "builtin-webhook",
    fieldID: "webhook-url",
    value: "http://google.com"
  })

Invalid webhook (returns false):

  destinationFieldValidate(input:{
    destType: "builtin-webhook",
    fieldID: "webhook-url",
    value: "missing-schema"
  })

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

Out of Scope:

• Search-select functionality will be in a subsequent PR.

Describe any introduced user-facing changes:
N/A

Describe any introduced API changes:

• Query.destinationTypes
• Query.destinationFieldValidate

Additional Info:
A full working implementation of the full suite of API and UI changes is in the generic-destinations branch.

Note: generic-destinations should be considered a POC for the purposes of flushing out the full scope of API changes, the UI code will be cleaned up and merged in pieces with tests.

[tony-tvu]

Functionally working as expected on my end.