Update ADK doc according to issue #1227 - 2 - MCP Toolset Authentication

[adk-bot]

2. Document MCP Toolset Authentication

Doc file: docs/tools-custom/mcp-tools.md

Current state:

Does not mention support for authentication configuration in McpToolset.

Proposed Change:

Add a section "Authentication for Remote MCP Servers".
Explain that McpToolset accepts auth_scheme and auth_credential parameters.
Explain that these are used to generate Authorization headers (Bearer, Basic, API Key) for the MCP connection (useful for SseConnectionParams).

Reasoning:
McpToolset has been updated to support auth_scheme and auth_credential and implements _get_auth_headers to inject them into the session.

Reference: src/google/adk/tools/mcp_tool/mcp_toolset.py

[zyantw]

Closing this in anticipation of this change merging:
#2081 and also duplicate of #1230