Authentication for engine api

[holiman]

Follow-up to #162 .

Differences:

• Added that we're using the 'next' port,
• Added section about claims, and added two more optional claims.

Todo:

• examples
• reference the authentication.md from the main page

[djrtwo]

looks good in general! left a few nits

[djrtwo]

spellcheck is complaining about all of the acronyms used here.

Seems like we need to add them to the wordslist -- https://github.com/ethereum/execution-apis/blob/main/wordlist.txt

[mkalinin]

Great work! I left a few nits and question. Also, a general thing, IMO we should add an ## Authentication section to the main specification and reference this document. This section may contain keypoints like CLI param, port, and a short description of the approach i.e. JWT reference.

[djrtwo]

This looks good to me. Merging in next 24 hours