Allow overriding connection class via keyword arguments

[m-novikov]

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Description of change

This allows overriding the connection class which is implicitly guessed by the schema provided in the URI.
While other arguments parsed from URI have precedence, current behavior is inconsistent, user can override class for default redis:// but not rediss:// and unix://.
Closes: #1739

[chayim]

@m-novikov Thank you for the contribution! We'll merge it in, and this will release with the upcoming release candidate.