feat(agentregistry): add package foundation and REST transport

[wolo-lab]

Problem:
adk-go has no client for the Google Cloud Agent Registry
(agentregistry.googleapis.com) — the governed catalog of A2A agents, MCP
servers, and model endpoints. adk-python already ships one, so we need a Go
equivalent with matching behavior to discover and construct agents/toolsets from
the registry.

Solution:
Add the agentregistry package foundation — everything the discovery
methods and the RemoteAgent/McpToolset factory helpers (follow-up PRs in this
stack) build on. This PR is additive: a new package, no changes to existing
public APIs.

• Client surface: Config, Client, and New — ADC-authenticated by
  default, or a caller-supplied *http.Client.
• REST transport: an authenticated Transport to
  agentregistry.googleapis.com, resolving resource paths (absolute
  projects/... vs. parent-relative), setting the x-goog-user-project quota
  header, and returning non-2xx responses as a typed *APIError. Transport is
  an interface seam so alternative transports can back the same Client.
• mTLS endpoint selection: honors GOOGLE_API_USE_MTLS_ENDPOINT
  (auto/always/never). In auto, the mTLS endpoint is chosen only when
  client-cert use is opted in (GOOGLE_API_USE_CLIENT_CERTIFICATE) and a
  default client-certificate source is actually present on the machine —
  matching adk-python / google-auth (has_default_client_cert_source) and
  avoiding targeting the mTLS endpoint with no client cert.
• Quota project: x-goog-user-project prefers the ADC credentials' quota
  project (GOOGLE_CLOUD_QUOTA_PROJECT → credentials JSON quota_project_id)
  and falls back to the configured ProjectID, matching adk-python's
  credentials.quota_project_id or project_id.
• Wire types & options: Agent, McpServer, Endpoint, Protocol,
  Interface, Skill, Card, the *Page list responses, and ListOptions
  (WithFilter / WithPageSize / WithPageToken).
• Resolution helpers (parity with adk-python): connectionURI
  (_get_connection_uri), cleanName (_clean_name), and the
  protocol-binding → A2A transport map (_TRANSPORT_MAPPING).