feat: add configs for external auth MCP usage + tool allow/denylist (#19794)

Closes https://github.com/coder/internal/issues/988

The logic for allowing/denying tools can be found in https://github.com/coder/aibridge/pull/4/files#diff-330a6371a583dd8cadeed79b95499e3a87960ad8ea4d6a94061e8f88a44834c3 (`ProxyBase.filterAllowedTools`).

codersdk/testdata/githubcfg.yaml

@@ -17,6 +17,9 @@ externalAuthProviders:
       - token
     device_flow: true
     device_code_url: https://example.com/device
+    mcp_url: https://api.githubcopilot.com/mcp/
+    mcp_tool_allow_regex: .*
+    mcp_tool_deny_regex: create_gist
     regex: ^https://example.com/.*$
     display_name: GitHub
     display_icon: /static/icons/github.svg