feat: add prefixes for chainid

[carsonfarmer]

As per our prior discussion. This now means that JWTs look something like this:

Header

{
  "alg": "ETH",
  "typ": "JWT",
  "kid": "eth:rinkeby:0x0000000..."
};

Payload

{
  "nbf": 12345,
  "iat": 12345,
  "exp": 56789,
  "iss": "eth:rinkeby:0x0000000...",
  "sub": "eth:rinkeby:0x0000000...",
  "aud": "0x111111111..."
}

I've used : as delimiters because these cannot show up in the address itself, so it is a safe delimiter to use. Additionally, it is similar to the did spec, though I've moved away from using "proper" key dids here because that adds complexity that we just don't need.

Note that aud is the address of the provider, and kid is the key id for the address. This is a non-standard JWT, with borrows ideas from ethereum/EIPs#1341. This can be validated for polygon and ethereum by extracting the address from the header, and using https://goethereumbook.org/signature-verify/ to verify that the signed payload was signed by the correct private key/address.

[carsonfarmer]

Tests should be passing now @asutula