Replace keys during array merge
Change namespace, make tests separate
Add interface replace class
#6903 - adding docblocks to the new `MergeReplaceKeyInterface`
#6903 - making `MergeReplaceKey` `final`, removing inherited docblock documentation
#6903 - importing `MergeReplaceKeyInterface` interface
#6903 - adding `@group` annotation for newly introduced tests
Merge branch 'feature/#6903-allow-replacing-keys-in-array-utils' into develop

Close #6903