Allow KeyLookup in Cookie for KeyAuth middleware

[FrontSide]

Issue Description

It would be great if we could specify a cookie name for KeyLookup in the KeyAuth middleware config.
This way a user could pass the key credentials via a cookie, whereas now it is restricted to http headers, query params, and form fields.

Disclaimer: There might be good - security-related - reasons not to do this.

Checklist

• Dependencies installed
• No typos
• Searched existing issues and docs

Expected behaviour

I can specify

KeyLookup: "cookie:<name>"

when defining KetAuthWithConfig, so the middleware reads the key from a cookie.

e.g.:

e := echo.New()
e.Use(middleware.KeyAuthWithConfig(middleware.KeyAuthConfig{
  KeyLookup: "cookie:api-key",
  Validator: func(key string, c echo.Context) (bool, error) {
               return key == "valid-key", nil
     },
}))

Actual behaviour

Currently only following KeyLookup methods are allowed

"header:<name>"
"query:<name>"
"form:<name>"	

Steps to reproduce

n.A.

Working code to debug

n.A.

Version/commit

v4.3.0/7846e3f

[lammel]

Could you provide a PR for that change? @FrontSide
Should be fairly easy to implement.