[Connector Builder] Use ref resolving endpoint before converting YAML to UI

[lmossman]

Context

YAML to UI conversion logic was added in #21142

and an endpoint to resolve refs and options was added in #21565

We would now like to wire up the connector builder to call that resolving endpoint before trying to convert the YAML to UI forms, so that YAMLs containing refs and options can be successfully converted (they are currently rejected).

Acceptance Criteria

• YAML manifests containing refs and options can be successfully converted to UI forms
• Invalid manifests show an error indicating that they are invalid and therefore cannot be converted

[lmossman]

Waiting on review