More tidying up of `aws-smithy-runtime-api` (#2869)

This PR finishes documenting `aws-smithy-runtime-api` and also:

- Adds `Send + Sync` bounds to `Interceptor`.
- Moves `Interceptors` into `aws-smithy-runtime`.
- Expands the more complicated macros in the interceptor context
wrappers.
- Makes the `OrchestratorError` an informative error according to
[RFC-22](https://github.com/awslabs/smithy-rs/blob/main/design/src/rfcs/rfc0022_error_context_and_compatibility.md).
- Renames `ConnectorError::is_other` to `as_other` and adds an
equivalent `is_other` that returns a boolean.

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