Add support for loading OpenAPI specs from local files.

[marioolf]

This PR adds the ability to load OpenAPI specifications from local files.

Changes:

• Updated fetch_swagger_spec function to check if input is a local file.
• In case it is a file, the tool parses the content.
• If it is not a file, the tool treats input as URL, maintaining the existing functionality.

Testing:

• Verified the functionality using sample openapi.yaml local file with valid OpenAPI spec.
• The tool loaded the file, parsed the spec and processed the endpoints.

This addresses the request described in issue #3