Improve support for x-form-urlencoded content usage with http:Client

[ayeshLK]

Purpose

$subject

Fixes #1705

Examples

Initially when sending x-form-urlencoded content via http:Client developers had to do additional work to generating the text payload. But with this improvement following is possible.

http:Response response = check snowpeak->post("/update", 
    {
        "key1": "value1",
        "key2": "value2"
    }, 
    mediaType=mime:APPLICATION_FORM_URLENCODED
);

Checklist

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

[codecov]

Codecov Report

Merging #797 (92bbefa) into master (3f716cc) will increase coverage by 0.13%.
The diff coverage is 97.36%.

@@             Coverage Diff              @@
##             master     #797      +/-   ##
============================================
+ Coverage     80.62%   80.76%   +0.13%     
  Complexity       71       71              
============================================
  Files           344      344              
  Lines         17615    17632      +17     
  Branches       3978     3990      +12     
============================================
+ Hits          14202    14240      +38     
+ Misses         2633     2618      -15     
+ Partials        780      774       -6     

Impacted Files	Coverage Δ
ballerina/http_commons.bal	88.03% <95.23%> (+0.47%)	⬆️
ballerina/http_client_endpoint.bal	88.53% <100.00%> (ø)
ballerina/resiliency_failover_client.bal	86.40% <100.00%> (ø)
ballerina/resiliency_load_balance_client.bal	87.59% <100.00%> (ø)
...a/stdlib/http/transport/message/MessageFuture.java	91.30% <0.00%> (-4.35%)	⬇️
...transport/message/Http2InboundContentListener.java	51.28% <0.00%> (-2.57%)	⬇️
...ib/http/api/client/actions/AbstractHTTPAction.java	68.67% <0.00%> (+0.80%)	⬆️
...dlib/http/transport/message/HttpCarbonMessage.java	94.81% <0.00%> (+1.03%)	⬆️
...ttp/transport/message/HttpMessageDataStreamer.java	89.77% <0.00%> (+2.27%)	⬆️
...impl/listener/http2/Http2ServerTimeoutHandler.java	80.82% <0.00%> (+2.73%)	⬆️
... and 4 more

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