feat: Add protocol support for extensions

[mikeas1]

Description

This updates the protocol to support Extensions. Extension profiles are documents that describe modifications or additions to the core A2A profile. AgentCards can declare supported extensions via capabilities.extensions. This declaration can include additional information via capabilities.extensions.params. Clients can use this as a basis for extension negotiation: if a client supports an extension that the server supports, the client can then follow the protocol defined by the extension. This can include, but is not specifically limited to:

1. Requiring specific structure of messages, such as following a specific schema for a DataPart.
2. Providing additional information in request/response Message and Artifact objects via their metadata field.
3. Exposing additional methods beyond the core A2A protocol methods.

Messages and Artifacts can now include an indication of which extensions are present in their content. I do not yet know if extension indications are necessary on other data structures (such as MessageSendParams or TaskStatus). I'd say extension indications are not strictly necessary at all: it's possible to infer that an extension was followed by inspecting the message content itself (are the metadata fields you expect present, does the DataPart follow the schema you expect, etc). However, they are useful for validation and simplifying extension implementation logic: "you said you followed this extension, so I'll validate fields are present and then perform my processing".

Fixes #585

[pstephengoogle]

Can we also update the grpc definitions accordingly?

[zeroasterisk]

https://a2a-protocol.org/latest/topics/extensions/
Doc link to help readers who end up on this pr