Support layer metadata register to annotate data objects/layers built from data directory

[eblondel]

Relates to #263 on multiple data/layer support for a single entity.

By default, N data objects will be created based on the datasources found in a data directory (eg. multiple geopackages, multiple geotiff), etc. The only information available to identify each particular source is the filename, which is inherited as layername. No other metadata information is handled, eg; human description that characterizes and distinguishes each data object/layer

The objective of this development is to complement the handling of multiple data sources (through a data directory approach) with layer metadata information, through a standardized register (following the geoflow register data structure code/uri/label/definition; as CSV file) that can be made available in the data directory.

When not available, the default naming for layers / online resources will be used (as of now). When available, these additional layer metadata will be recycled to annotate the layers/online resources. This aims to provide meaninful domain description of multi layers associated with a dataset;

cc @helraynal

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