Berry webserver.header to read browser sent headers

[s-hadinger]

Description:

Berry add webserver.header(name: string) -> string or nil return the header with key name sent by the browser. The name is case sensitive. Return nil if the header is not present or not collected. Currently only Referer, Host, Authorization and If-None-Match.

import webserver
var host = webserver.header("Host")
print(host)

Checklist:

• The pull request is done against the latest development branch
• Only relevant files were touched
• Only one feature/fix was added per PR and the code change compiles without warnings
• The code change is tested and works with Tasmota core ESP8266 V.2.7.4.9
• The code change is tested and works with Tasmota core ESP32 V.2.0.14
• I accept the CLA.

NOTE: The code change must pass CI tests. Your PR cannot be merged unless tests pass