Add a way to retrieve a sender

[fraguada]

Step 0:

• I've read the contribution guidelines!

Expected Behaviour

I can give read/write permissions to someone for messing with a stream. I would expect that someone to be able to write to that stream.

Actual Behaviour

No exposed way for someone to write to that stream if they don't have the source (sender) client saved somewhere (3dm file user data for example)

Affected Projects

Does this issue propagate to other dependencies or dependants? If so, list them here!

SpeckleRhino

Reproduction Steps & System Config (win, osx, web, etc.)

Proposed Solution (if any)

Add a way to retrieve a sender via ID