cli: allow validating a configuration file

Direct user feedback.

The cli should allow validating a configuration file the user intends to feed to mcpjungle server.

examples:

mcpjungle validate ./my_server.json

mcpjungle validate ./my_tool_group.json

mcpjungle validate ./new_mcp_client.json

Maybe the user also needs to specify the type of configuration they're validating (mcp server, client, etc) or maybe mcpjungle can infer the type based on certain rules (eg- if url is present, this is probably a http mcp server). Needs to be determined.

The validation logic should be implemented server-side so that the validation feature can be consumed by any client (CLI for now, frontend in future).

This validation logic can also be used by backend during POST / PUT requests.

This should also be mentioned in the docs. Originally, a user was trying to search for this functionality in the docs. So it should be discoverable from there.