chore(notif-platform): More utility on NotificationProvider

[leeandher]

Adds a few things to NotificationProvider

• get_renderer() -> uses the default renderer by default
• target_class -> must be a subclass of NotificationTarget, added a test for this
• target_resource_types -> a list of valid resources that this provider can consume
• validate_target -> a method for the eventual NotificationService to validate the incoming target

The NotificationTarget classes are dataclasses with their own validation. Even though it's a side-effect, I think it's reasonable to prevent initializing a dataclass which will produce an error if used, though I added a todo about a making needless RPC calls

All of the validation has added tests, and the renderers/providers being stubbed out have also been expanded.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

:white_check_mark: All tests successful. No failed tests found.

Additional details and impacted files

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##           master   #93562      +/-   ##
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+ Coverage   85.65%   88.03%   +2.38%     
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  Files       10331    10336       +5     
  Lines      596435   596434       -1     
  Branches    23161    23124      -37     
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+ Hits       510869   525088   +14219     
+ Misses      85107    70853   -14254     
- Partials      459      493      +34