Add option to preserve refs in served spec

[chbndrhnns]

Fixes #2127.

Changes proposed in this pull request:

• Added SwaggerUIOptions(resolve_spec_refs: bool = True) in connexion/options.py:40.
• Updated the served spec endpoint to return raw when resolve_spec_refs=False in connexion/middleware/swagger_ui.py:72.
• Added tests covering the default expanded behavior and the new preserved-ref behavior in tests/api/test_bootstrap.py:124.

[coveralls]

coverage: 94.298% (+0.01%) from 94.288% — chbndrhnns:feature/configurable-served-spec-refs into spec-first:main