Allow users to set HTTP Connectors in Configs

[Velfi]

Refactor our current network stack to allow users to set HTTP Connectors in Service Configs and Shared Configs.

Then, update Service Client creation to use this connector instead of creating a connector from defaults based on the currently enabled features.

This is in support of the "How we should use features" RFC.

[jdisanti]

The connector customization needs to play into network calling credential providers (e.g., IMDS, HTTP token, etc) provider initialization as well: awslabs/aws-sdk-rust#579

[jdisanti]

This would probably be implemented along side #960

[Velfi]

This is partly implemented by #1918

[jdisanti]

This was included in release-2022-12-14.