feature/mx-1387 update identity provider with assign method

[cutoffthetop]

• replace mex.common.identity.query functions with ~.registry
  • register_provider(...) allows mex-extractors and mex-backend to register their own IdentityProvider implementations
  • get_provider() gets an instance of the currently configured id provider
• update function signatures of BaseProvider to cut sqlite ties
• rename dummy provider to memory
• deprecate mex.common.models.base.FlatValueMap - that was dead code
• deprecate the bw-compatible properties of mex.common.identity.models.Identity - like merged_id
  • we need to use their proper names now (that comply with mex-model) - like stableTargetId

[mr-kamran-ali]

Nice, like in code documentation and explanations. But ticket ACs are still not met, are you gonna take care of this in another branch?: "add backend identity connector in mex-common to get Item.identifier and item.stableTargetID from backend"

[cutoffthetop]

Nice, like in code documentation and explanations. But ticket ACs are still not met, are you gonna take care of this in another branch?: "add backend identity connector in mex-common to get Item.identifier and item.stableTargetID from backend"

yes, there's gonna be more branches. but in the backend and extractor repos ;)