Add `const` strings for supported OpenID4VC standard versions

[nanderstabel]

Description

As title.

These const values can be used downstream to keep track of the OpenID4VC standard versions that this library supports.

Motivation

Easier and automatic tracking of protocol versions.

Resources

n/a

To-do List

• Add const values

[JelleMillenaar]

Make it an array so we can also support multiple versions ;)