feat(keys): add a new field x5t to the entity keys

[catbro666]

Summary

jwe-decrypt plugin wants to support searching key by x5t, so there's this PR. This PR adds a new field x5t for the keys entity. And similar to the kid, the x5t and set_id combination should be unique.

One customer is using Azure to generate JWE token and the protected header of a JWE token generated by Azure is as below:

{
"alg": "RSA-OAEP",
"enc": "A128CBC-HS256",
"x5t": "I67Q-7SSeuHsXgrpMq_K_3idefI",
"zip": "DEF"
}

It uses x5t field instead of kid field and the enc is A128CBC-HS256. So customer is requesting jwe-decrypt plugin to support searching key by the x5t field.

Checklist

• The Pull Request has tests
• A changelog file has been created under changelog/unreleased/kong or skip-changelog label added on PR if changelog is unnecessary. README.md
• There is a user-facing docs PR against https://github.com/Kong/docs.konghq.com - PUT DOCS PR HERE

Issue reference

https://konghq.atlassian.net/browse/FTI-6399