DecideBackendByRequester: add default_backend setting

[vladimir-mencl-eresearch]

Enhance the DecideBackendByRequester microservice: support a default_backend setting - to avoid having to spell out all requesters: with default backend. With a default backend, only exceptions/overrides need to be listed.

Also add missing tests for DecideBackendByRequester - and then add tests covering this feature.

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[peppelinux]

I completely agree, that was the previous default behaviours before the refactoring

[c00kiemon5ter]

Thank you!