Implement the Aggregate Module of the Morphir SDK

Is your feature request related to a problem? Please describe.
Morphir's SDK seeks to have parity with much of elm/core and should have full parity with elm/core Maybe.

Describe the solution you'd like
Implement the functions in [elm/core Aggregate](https://github.com/finos/morphir-elm/blob/main/src/Morphir/SDK/Aggregate.elm) and add tests. Over time we will add more functionality here, but this is a good start.

Describe alternatives you've considered
N/A

Additional context
N/A

Start by reviewing the linked elm/core Aggregate implementation, then locate the corresponding Aggregate module and test area in morphir-dotnet. Implement the requested Aggregate functions and add tests demonstrating parity with the reference; done means the module behavior is covered by passing tests.

Written by the indexing model from the issue text.