feat(openapi): create openapi toolset

[AmaadMartin]

Link to Issue or Description of Change

Or, if no issue exists, describe the change:

Problem:
The adk-js repository lacked the ability to automatically generate tools from OpenAPI specifications, requiring developers to manually define tools for REST APIs and limiting parity with the Python implementation.

Solution:
Implemented the OpenAPI Toolset feature, adding a complete suite of tools and parsers to handle OpenAPI specifications:

• OpenAPIToolset (openapi_toolset.ts): A new toolset class that parses an OpenAPI specification and generates a set of RestApiTool instances.
• RestApiTool (rest_api_tool.ts): A generic tool capable of executing REST API calls, mapping arguments to path, query, header, and body parameters.
• OpenApiSpecParser: Includes sanitizeSchemaTypes to ensure schema compatibility.
• ToolAuthHandler: Includes a credential store in the session state for caching resolved credentials.

Testing Plan

Unit Tests:

• I have added or updated unit tests for my change.
• All unit tests pass locally.

Summary of passed results:

• Added comprehensive unit tests for all new files, achieving an overall coverage of 95.14% for the openapi_tool directory.
• Actual Line Coverage by File:
  • auth/auth_helpers.ts: 100%
  • openapi_spec_parser/tool_auth_handler.ts: 100%
  • auth/credential_exchangers/auto_auth_credential_exchanger.ts: 100%
  • rest_api_tool.ts: 95.78%
  • openapi_spec_parser/openapi_spec_parser.ts: 94.81%
  • openapi_spec_parser/operation_parser.ts: 93.60%
  • openapi_toolset.ts: 93.58%
  • auth/credential_exchangers/service_account_exchanger.ts: 93.54%

Integration Tests:

• All integration tests pass locally.
• Verified that OpenAPIToolset can parse sample specs and execute tools with mocked fetch.

Checklist

• I have performed a self-review of my own code.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.