feat: Add way for 3rd party resources to register into the PMR cli.

[DanCardin]

I plan to add documentation about this, once I make the experience of defining a custom Config class much more like defining a "basic" dataclass, than it currently is.

However, as-is, this change allows https://github.com/dancardin/responsaas to auto-register into the PMR cli, which i appreciate.

[coveralls]

Pull Request Test Coverage Report for Build 7743812980

• -12 of 21 (42.86%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall first build on dc/plugin at 89.674%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/pytest_mock_resources/cli.py	1	4	25.0%
src/pytest_mock_resources/plugin.py	8	17	47.06%

Totals
Change from base Build 7743789567:	89.7%
Covered Lines:	1428
Relevant Lines:	1575

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💛 - Coveralls