[Enhancement]: Add TIN Format Enum

[JSIV]

Description of Enhancement :

Enumerate some common TIN

Why is this Needed?

Current Tax ID is unstructured string. At least trying to capture what format it represents would improve compatibility.

Anything else we need to know?

[JSIV]

TWG Notes

1. Create an enum
2. Add an optional unstructured description field required if enum is OTHER

[JSIV]

We want to see if we can add this without making a breaking change. Should be doable. If not, let's put it in the notes. for v2

[Sumukh Sridhara]

I played around with a structured format for US Tax IDs, but all it would do is validate against a regex.

[Patrick Johnmeyer]

Closed per TWG meeting: #475 (comment)