Union semantics and variant enumeration order for tag bits

Currently, the spec says:

https://abs-tudelft.github.io/tydi/specification/logical.html#union-semantics

```
The "tag" field is used to represent which variant is being represented, by means of a zero-based index encoded as an unsigned binary number.
```

Do we want to allow custom tag enumeration of union variants?
If yes, we should add this to the spec.
If no, we should make explicit that the current text refers to the field index in the union declaration.
In the example, the tag enumeration is done by order of variant appearance in the union declaration, which seems most logical and straightforward, and it is similar to what most programming languages do as far as I am aware.

Read the union semantics section in the specification at the provided URL. Understand how union variants and tag bits are currently defined. Examine the existing spec text and examples to see if custom tag enumeration is allowed. Determine whether to clarify the current rule or add support for custom enumeration, then propose a spec update.

Escrito por el modelo de indexación a partir del texto del issue.