Go binding: Finish implementation of the String module

## Go binding: Finish implementation of the String module

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

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

The implementation should include all the functionality for String 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/