config: make service configs configurable

[mirekys]

This enables users of the module to override default configs of the provided services.

One could implement & provide their own ServiceConfig class inheriting from CommunityServiceConfig,
and e.g. customize PermissionPolicies, record links, add fields to Community records through
extending record schema, etc.

[lnielsen]

First of all, thanks for the contribution and apologies I didn't see this earlier.

We're trying to avoid the pattern used here as it opens up too large an API that we're likely not going to be able to keep stable, and often the configs might be interconnected making it hard for users of InvenioRDM to change it.

Please see the example here:
https://inveniordm.docs.cern.ch/develop/topics/service/#instantiating-a-service

service = ClickService(ClickServiceConfig.build(current_app))
service.click(system_identity)

You'll find the pattern implemented in Invenio-RDM-Records - e.g. https://github.com/inveniosoftware/invenio-rdm-records/blob/master/invenio_rdm_records/services/config.py#L99-L102

Which specific items do you want to configure? Just permissions?