[Fn::FindInMap] - [Coverage] - Add support for intrinsic functions such as Fn::Join

### Name of the resource

AWS::ElasticLoadBalancingV2::LoadBalancer

### Resource name

Fn::FindInMap

### Description

Request for intrinsic functions to be supported in the FindInMap function. Even though we can reference parameters in the function, it is not possible to customize the parameters of the FindInMap function, for example, allowing the appending of characters to a value in a parameter as shown below in the third parameter:
!FindInMap [parameter1, !Ref parameter2, !Ref parameter3,"appendedLetters"]
Unable to use !Join as it is not supported in FindInMap:
!FindInMap [parameter1, !Ref parameter2, !Join [“”,[!Ref parameter3,“appendedLetters”]]]

The following error occurs:
Template error: every Fn::FindInMap object requires three parameters, the map name, map key and the attribute for return value

### Other Details

_No response_

No repository files, tests, or implementation entry points are identified in the issue. Start by reviewing how Fn::FindInMap and nested intrinsic functions are represented and validated in the project, then determine the scope of supporting Fn::Join within FindInMap; done means the requested expression is accepted and its resulting value is covered by tests.

Written by the indexing model from the issue text.