The repositories API didn't allow any access to the extra_data field,

either reading from the field or setting values in the field. This made

it difficult to look up important settings that might affect a client,

or to set any custom values.
This change exposes the field to the caller, allowing them to see all

the settings (custom and tied to a SCMTool/hosting service), and to

modify it using standard means (directly setting fields or using JSON

Patches or Merge Patches).

All unit tests pass.
Tested by a customer who was building an integration that needed this.

Verified on their end that the fields in extra_data could be safely

manipulated.