chore: refactor connector api so that an async variant can be created

[sighphyre]

Restructures the connector APIs to be a bit cleaner and so that we can add in async variants of these. Effectively this only moves the load_features logic from the abstract base class into a pure function so that it can be reused by async connectors, since those will need their own ABC

[coveralls]

Pull Request Test Coverage Report for Build 19061071890

Details

• 47 of 56 (83.93%) changed or added relevant lines in 8 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.3%) to 93.352%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
UnleashClient/connectors/base_sync_connector.py	13	15	86.67%
UnleashClient/connectors/streaming_connector.py	3	5	60.0%
UnleashClient/connectors/hydration.py	15	20	75.0%

Totals
Change from base Build 19028210296:	-0.3%
Covered Lines:	674
Relevant Lines:	722

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💛 - Coveralls