satosa-saml-metadata script could be more flexible wrt output file names

[dnmvisser]

Currently the satosa-saml-metadata script creates a metadata file called backend.xml, or, when creating split metadata files, filenames that include an index number:

https://github.com/IdentityPython/SATOSA/blob/c629dd5dfce44675bc263241c880a6481ccb33d6/src/satosa/scripts/satosa_saml_metadata.py#L23

So, when one starts to use --split-backend (or --split-frontend), this will cause a change in the generated filenames, which can be problematic as other system might rely on them (aggregator tools etc).

The script could have some options to influence the output file names. Some ideas:

• An option to override the name of the first file
• Include the name of the backend/frontend in the file name

[c00kiemon5ter]

I am planning to do a rewrite but it is not very high on my list. The idea is to switch to poetry, use typer, rework the plugin loader and routing and then simplify how the options are set and add options to specify output names/patterns/strategies.