feat: add a2a liteagent, auth, transport negotiation, and file support

* feat: add server-side auth schemes and protocol extensions

- add server auth scheme base class and implementations (api key, bearer token, basic/digest auth, mtls)
- add server-side extension system for a2a protocol extensions
- add extensions middleware for x-a2a-extensions header management
- add extension validation and registry utilities
- enhance auth utilities with server-side support
- add async intercept method to match client call interceptor protocol
- fix type_checking import to resolve mypy errors with a2aconfig

* feat: add transport negotiation and content type handling

- add transport negotiation logic with fallback support
- add content type parser and encoder utilities
- add transport configuration models (client and server)
- add transport types and enums
- enhance config with transport settings
- add negotiation events for transport and content type

* feat: add a2a delegation support to LiteAgent

* feat: add file input support to a2a delegation and tasks

Introduces handling of file inputs in A2A delegation flows by converting file dictionaries to protocol-compatible parts and propagating them through delegation and task execution functions. Updates include utility functions for file conversion, changes to message construction, and passing input_files through relevant APIs.

* feat: liteagent a2a delegation support to kickoff methods

Author: greysonlalonde

lib/crewai/src/crewai/a2a/auth/__init__.py

@@ -1,20 +1,36 @@
 """A2A authentication schemas."""
 
-from crewai.a2a.auth.schemas import (
+from crewai.a2a.auth.client_schemes import (
     APIKeyAuth,
+    AuthScheme,
     BearerTokenAuth,
+    ClientAuthScheme,
     HTTPBasicAuth,
     HTTPDigestAuth,
     OAuth2AuthorizationCode,
     OAuth2ClientCredentials,
+    TLSConfig,
+)
+from crewai.a2a.auth.server_schemes import (
+    AuthenticatedUser,
+    OIDCAuth,
+    ServerAuthScheme,
+    SimpleTokenAuth,
 )
 
 
 __all__ = [
     "APIKeyAuth",
+    "AuthScheme",
+    "AuthenticatedUser",
     "BearerTokenAuth",
+    "ClientAuthScheme",
     "HTTPBasicAuth",
     "HTTPDigestAuth",
     "OAuth2AuthorizationCode",
     "OAuth2ClientCredentials",
+    "OIDCAuth",
+    "ServerAuthScheme",
+    "SimpleTokenAuth",
+    "TLSConfig",
 ]