Add HTTP basic and bearer auth support to the orchestrator (#2622)

## Motivation and Context
This PR adds support for Smithy's `@httpBasicAuth` and `@httpBearerAuth`
auth schemes, and ports the `@httpApiKeyAuth` scheme to the
orchestrator. This is prerequisite work for supporting Amazon
CodeCatalyst since that requires bearer auth.

This PR also fixes a bug in auth orchestrator that caused an error if no
identity is present for a scheme even when an identity for a lower
priority scheme is available.

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Co-authored-by: Zelda Hessler <zhessler@amazon.com>