Tidy up `aws-smithy-runtime-api` a bit (#2867)

This PR makes some progress towards documenting the
`aws-smithy-runtime-api` crate, as well as some additional work to make
it more stable:

- Places the `client` module behind a `client` feature so that it will
be possible to add a `server` module/feature in the future.
- Deletes `ConfigBagAccessors`.
- Renames auth types to reflect changes in the internal spec.
- Moves `RequestAttempts` into the `client::retries` module.
- Moves several types that were in floating around in
`client::orchestrator` to their own modules.
- Renames `Connector` to `HttpConnector`.
- Changes `DynResponseDeserializer` to `SharedResponseDeserializer` for
consistency with serialization, and also so that it could be moved into
runtime components or config in the future (since those need to
implement `Clone`).
- Updates the identity and auth design doc.
- Updates READMEs and crate-level documentation.
- Fixes most, but not all, the missing documentation in the crate.
- Hides the builder macros.

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