Allow messaging.system to be extended by system-specific definitions

[pyohannes]

#517 changed messaging.system to an open enum. This currently requires an exhaustive list of values to be managed as part of generic messaging semantic conventions, all possible values have to be defined as part of the attribute definition.

@lmolkova proposed an improvement to this situation, which would allow system-specific definitions to extend an existing open enum. This would provide clear ownership of value definitions.

This should be applied for messaging.system as soon as it is supported by the build tools.

[pyohannes]

Closed as a duplicate of #670.