💡 ETag header / ctx.Fresh

[Fenny]

This is a proposal to finish the ctx.Fresh() method along with ETag header support.

• The Fresh method originates from expressjs/req.fresh but we never finished it context#fresh

• When the response is still “fresh” in the client’s cache true is returned, otherwise false is returned to indicate that the client cache is now stale and the full response should be sent.
  When a client sends the Cache-Control: no-cache request header to indicate an end-to-end reload request, this module will return false to make handling these requests transparent HTTP/1.1 Caching Specification.

• Express uses the jshttp/fresh module, source code MIT.
  I found a Go net/http version from go-http-utils MIT

• It would be a good idea to implement ETag support within the Fiber core before starting on the Fresh method.
  jshttp/etag MIT
  go-http-utils/etag MIT
  expressjs/send-etag
  expressjs/weak-etag
  expressjs/strong-etag

Let's add this to the project board 💪 @gofiber/maintainers