Add API to EventSource for reading the reference metadata

[maxnoe]

Please describe the use case that requires this feature.

Add the moment, the Provenance system tries to read the input reference metadata itself in add_input_file, however,
this makes it very inflexible and violates in some ways the plugin architecture.

We should add an API to the EventSource to obtain the reference metadata, so that plugins can support reading the reference metadata for arbitrary inputs.

Describe the solution you'd like

Add a new property:

@property
@abstractmethod
def reference_metadata(self) -> ReferenceMetadata:

to the EventSource.

Describe alternatives you've considered

Alternative would be that we either
a) implement reading of reference metadata for all expected file types in ctapipe itself
or
b) live with missing input reference metadata for plugin event sources

both seem worse than adding this API.