Go binding: Finish implementation of the Key module

[michelchan]

Go binding: Finish implementation of the Key module

Item	Detail
Binding	Go
Module	Morphir.SDK.Key
Relevant Morphir.IR.SDK module	Morphir.IR.SDK.Key
Relevant morphir-jvm code	morphir.sdk.Key
Relevant Elm Module	[Key](https://github.com/finos/morphir-elm/blob/main/src/Morphir/SDK/Key.elm
)

Provide a Go lang implementation of the Morphir.SDK.Key module for Morphir.SDK.

The implementation should include all the functionality for Key as well as tests (we are willing to have this split across PRs).

Please place it here: https://github.com/finos/morphir/tree/features/polyglot-morphir/corelib/go/morphir/sdk/