Add OAuth2 JWT bearer grant type support for client

[ldclakmal]

Purpose

This PR adds OAuth2 JWT bearer grant type support for HTTP client.

Fixes: ballerina-platform/ballerina-library#1788

Examples

import ballerina/http;
import ballerina/io;

http:Client securedEP = check new("https://localhost:9090",
    auth = {
        tokenUrl: "https://localhost:9445/oauth2/token",
        assertion: "eyJhbGciOiJFUzI1NiIsImtpZCI6Ij[...omitted for brevity...]",
        clientId: "FlfJYKBD2c925h4lkycqNZlC2l4a",
        clientSecret: "PJz0UhTJMrHOo68QQNpvnqAY_3Aa",
        scopes: ["admin"],
        clientConfig: {
            secureSocket: {
                cert: "../resource/path/to/public.crt"
            }
        }
    },
    secureSocket = {
        cert: "../resource/path/to/public.crt"
    }
);

public function main() returns error? {
    string response = check securedEP->get("/foo/bar");
    io:println(response);
}

Checklist

• Linked to an issue
• Updated the changelog
• Added tests

[codecov]

Codecov Report

Merging #588 (f6bca0d) into master (aa4002f) will increase coverage by 0.15%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master     #588      +/-   ##
============================================
+ Coverage     80.13%   80.28%   +0.15%     
  Complexity       67       67              
============================================
  Files           335      335              
  Lines         17081    17082       +1     
  Branches       3816     3816              
============================================
+ Hits          13688    13715      +27     
+ Misses         2651     2630      -21     
+ Partials        742      737       -5     

Impacted Files	Coverage Δ
ballerina/auth_types.bal	100.00% <100.00%> (ø)
...transport/message/Http2InboundContentListener.java	51.28% <0.00%> (-2.57%)	⬇️
...tdlib/http/transport/contractimpl/common/Util.java	83.03% <0.00%> (ø)
...ib/http/api/client/actions/AbstractHTTPAction.java	67.46% <0.00%> (+0.80%)	⬆️
...nsport/contractimpl/DefaultHttpResponseFuture.java	74.67% <0.00%> (+1.94%)	⬆️
...dlib/http/transport/message/HttpCarbonMessage.java	95.08% <0.00%> (+2.18%)	⬆️
...ttp/transport/message/HttpMessageDataStreamer.java	89.77% <0.00%> (+2.27%)	⬆️
...ransport/message/DefaultFullHttpMessageFuture.java	100.00% <0.00%> (+4.00%)	⬆️
...contractimpl/listener/states/ReceivingHeaders.java	53.06% <0.00%> (+4.08%)	⬆️
...a/stdlib/http/transport/message/MessageFuture.java	95.65% <0.00%> (+4.34%)	⬆️
... and 3 more

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