Is your feature request related to a problem? Please describe.

#251 introduced validation of transports in the AgentCard vs what is on the classpath. While this works fine for projects under our control, it might cause problems for implementors.

Also, it isn't really mandated by the spec, so we should provide a mechanism to turn it off.

Describe the solution you'd like

Initially we'll provide a set of system properties:

• io.a2a.transport.skipValidation - Turns off validation for all transports
• io.a2a.transport.jsonrpc.skipValidation - Turns off validation of the JSONRCP transport
• io.a2a.transport.grpc.skipValidation - Turns off validation of the GRPC transport
• io.a2a.transport.rest.skipValidation - Turns off validation of the HTTP+JSON transport

Describe alternatives you've considered

A more flexible alternative than system properties could be to use MicroProfile Config. However, that might not be available in all environments.

Additional context

No response

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