Specify additional HTTP status codes

In the [HTTP(S) binding](https://identity.foundation/did-registration/#https-binding), we currently use a few HTTP status codes such as **200**, **201**, **400**, **500**.

Suggested by @cihanss, it would probably be useful to make use of additional HTTP status codes in certain cases, e.g.:
- **401** or **402** if `didState.action="getVerificationMethod"`
- **402** if payment is required for DID methods such as did:btcr
- **303** if `didState.action="redirect"`
- etc.