feat: add extensions attribute to ClientCapabilities and ServerCapabilities #581

Job	Run time
Server Conformance	1m 17s
Auth Conformance (auth/metadata-var3)	1m 14s
Client Conformance (tools_call)	1m 23s
Auth Conformance (auth/scope-from-www-authenticate)	7m 51s
Client Conformance (elicitation-sep1034-client-defaults)	1m 20s
Auth Conformance (auth/basic-cimd)	1m 11s
Auth Conformance (auth/metadata-default)	1m 3s
Client Conformance (sse-retry)	1m 13s
Client Conformance (initialize)	52s
Auth Conformance (auth/scope-retry-limit)	1m 3s
Auth Conformance (auth/token-endpoint-auth-basic)	1m 6s
Auth Conformance (auth/scope-step-up)	1m 3s
Auth Conformance (auth/token-endpoint-auth-post)	1m 12s
Auth Conformance (auth/token-endpoint-auth-none)	1m 1s
Auth Conformance (auth/scope-from-scopes-supported)	7m 54s
Auth Conformance (auth/pre-registration)	1m 0s
Auth Conformance (auth/scope-omitted-when-undefined)	1m 15s
Auth Conformance (auth/metadata-var1)	51s
Auth Conformance (auth/metadata-var2)	1m 14s
	35m 3s

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