Callable argspec validation should allow kwargs for kdim arguments

[philippjfr]

When writing DynamicMap callbacks you sometimes want to write a general callback which will work with different numbers of key dimensions. Currently the Callable argspec validation completely prevents this because each key dimension needs to be explicitly named in the callback signature. This makes it completely impossible (or at least very painful) to write a library level callback that handles different numbers of dimensions. I'd very strongly argue that using kwargs should lift the restrictions on validation as it already does for stream parameters.

[jlstevens]

I am happy to change policy so this style is not disallowed. It makes validation weaker when used so I in turn would strongly recommend not using this style unless necessary. But I do agree we shouldn't disallow it.

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