Move source-related queries from `Invoke()` function into Source.
This is an effort to generalizing tools to work with any Source that
implements a specific interface. This will provide a better segregation
of the roles for Tools vs Source.
Tool's role will be limited to the following:
* Resolve any pre-implementation steps or parameters (e.g. template
parameters)
* Retrieving Source
* Calling the source's implementation
Along with these updates, this PR also resolve some comments from
Gemini:
* update `fmt.Printf()` to logging as a Debug log -- within
`GetOperations()`
* update `fmt.Printf()` during failure to retrieve user agent into
throwing an error. UserAgent are expected to be retrieved successfully
during source initialization. Failure to retrieve will indicate a server
error.
0202709efc