fix: make goth and ex_aws_auth optional dependencies

[davydog187]

Summary

Closes #798.

goth and ex_aws_auth were required dependencies, but they're only used by two providers:

• google_vertex — Goth, for Google Cloud OAuth (ADC / refresh tokens / workload identity / metadata server)
• amazon_bedrock — ex_aws_auth, for AWS Signature V4 signing (IAM credentials, STS, bidi streaming)

Every user inherited both regardless of which provider they used. This marks them optional: true so they're only pulled in when explicitly added.

The non-obvious part

optional: true alone is not sufficient. Provider modules are compiled into :req_llm and discovered at runtime (ReqLLM.Providers.discover_providers/0), so the Bedrock/Vertex modules must still compile when a downstream user omits the optional dep. They referenced %AWSAuth.Credentials{} and %Goth.Token{} as compile-time struct literals/patterns, which raise a CompileError (__struct__/1 is undefined) when the dep is absent.

These are rewritten to plain-map form — %{__struct__: Mod, ...} for pattern matches and struct(Mod, ...) for construction — so they no longer force the module at compile time. Remote function calls (AWSAuth.sign_authorization_header, Goth.Token.fetch, etc.) compile fine without the dep and only need runtime guarding.

Changes

• mix.exs — both deps marked optional: true.
• Converted all compile-time struct references across amazon_bedrock.ex, amazon_bedrock/sts.ex, bedrock/bidi_stream.ex, google_vertex/auth.ex.
• Added ensure-loaded guards that raise a clear "add this dependency" error:
  • ensure_ex_aws_auth!/0 at Bedrock credential-resolution time (the pre-existing guard fired too late — after AWSAuth.Credentials.from_map/from_env was already called).
  • Code.ensure_loaded(Goth.Token) on the Vertex ADC/metadata path.
• Updated stale error-message version text (~> 1.3 → ~> 1.4).
• README: added an "Optional Provider Dependencies" section.

Verification

• mix compile + Bedrock/Vertex test suites: 180 tests, 0 failures. mix format --check-formatted and mix credo clean on changed files.
• Dep-less downstream check: a throwaway consumer project depending on this branch without goth/ex_aws_auth compiles cleanly (only xref warnings, no CompileError), neither dep is fetched, both providers still appear in ReqLLM.Providers.list/0, and exercising an IAM/ADC path raises the friendly dependency error rather than a raw UndefinedFunctionError.

Known gap

The bidirectional-streaming path (bidi_stream.ex, Nova Sonic) constructs credentials without an upfront guard, so it would raise a raw UndefinedFunctionError rather than the friendly message if ex_aws_auth is missing. Left as-is since it's a niche path — happy to add a guard for symmetry if preferred.