mcp: allow attaching resources as chat contextgit

[connor4312]

• Expose MCP server capabilities in IMcpServer to know what they can do
• Move MCP resource URIs to be hex encoded to avoid problems as the URI authority is case-insensitive and base64 is not.
• Add a helper for cancellation tokens with async iterables
• Let "add context" picks and contributions be AsyncIterable. Fixed an issue where its disposables would be leaked if the picker was closed before its promise resolved.
• Add a workbench contribution for registering resource picks to Add Context

cc @jrieken for Add Context changes