Introduce support for audited_class setting per model

[amkisko]

Done:

• test coverage for all changes in this PR
• README updated accordingly
• generators updated to support custom table name

Background:

• single table might become problematic for large amount of logs/audits
• partitioning of audits table leads to other problems to solve
• having full separation of audits tables per model gives more flexibility
• keeping it strict and as much verbose as needed
• compatibility with previous behavior required
• looked at Define audit_class per model #641

[amkisko]

@danielmorrison Hey! These changes has been tested in production for past two weeks, no issues found so far, works quite well.

[amkisko]

@danielmorrison CI tests are all actually green, there is this "missing coverage" assessment, which does not return anything locally (meaning that all changes are covered), but here it returns different results depending on Rails version and database driver.

[amkisko]

Any suggestions how to replicate it locally?

[amkisko]

There might be a bug introduced due to custom serializer, we have not figured out a test case for this one yet: undefined method 'merge' for an instance of String (NoMethodError) combine_target.audited_changes = audits_to_combine.pluck(:audited_changes).reduce(&:merge)

[amkisko]

@danielmorrison I switched from .pluck(:audited_changes) to .map(&:audited_changes) -- assumption here is that something has changed in Rails 7 in relation to column value serialization (could not find any evidences yet). It should not bring impact on performance as combine most probably have only few records during single request (basically previous request already did combine if max threshold crossed), but it's more clear what it does, pluck on ActiveRecord proxy does select of raw value.