Deprecate Content-Transfer-Encoding per RFC 8951

[61131]

The Content-Transfer-Encoding for EST has been deprecated as per RFC 8951. This document identifies that the POST request and payload response of all endpoints use base64 encoding. This format should be used regardless of any Content-Transfer-Encoding header, and any value in such a header MUST be ignored.

This PR removes the requirement for the Content-Transfer-Encoding header in requests and responses and any validation associated with the header if it is present.

[toddgaunt-gs]

Hey @61131 Thanks for submitting this pull request! I just wanted to let you know that I'll assign myself and another dev as reviewers since this seems like a nice change to follow the latest EST standards and I'd be happy to merge this functionality post-review.