[HTTP] Make the Host header overwritable

[TharmiganK]

Description:

Currently if the user intentionally add the Host header, we are overwriting it with the default host provided in the client URL. This is because the Host header has its own semantics in HTTP domain[1] and since in it is a not recommended to allow overwriting it programatically[2].

[1] https://www.rfc-editor.org/rfc/rfc9110#section-7.2
[2] https://developer.mozilla.org/en-US/docs/Glossary/Forbidden_header_name

Describe your problem(s)

import ballerina/http;
import ballerina/io;

public function main() returns error? {
    http:Client httpEp = check new (url = "https://sarinda.requestcatcher.com/test");
    string getResponse = check httpEp->/.get(headers = {"host":"sarinda.choreo.dev","host1":"sarinda.host1.choreo.dev"});
    io:println(getResponse);
}

Tracelogs for request:

GET /test/ HTTP/1.1
host1: sarinda.host1.choreo.dev
host: sarinda.requestcatcher.com
user-agent: ballerina
connection: keep-alive 

Describe your solution(s)

Let the user overwrite the Host header.