Support for converting headers to custom claims

[onematchfox]

Proposed solution to #15

PR is unfortunately re-based off #14 so the changes are slightly hidden. But all changes are contained in f232894

The meat of the change is the code at https://github.com/onematchfox/kong-upstream-jwt/blob/efb84b13caed2fcf508a299f6562049cfb82eb5e/src/access.lua#L127-L133. Additional changes include the additional schema options headers_to_claims and clear_headers_to_claims as well as README updates.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[onematchfox]

See #14 (comment)