adding support for passing request headers

In the #115 pull request, @sanketmehta7 proposed this feature but only for android and his pull request is blocked and not mergeable.

I am needing this support for one of my application both in android and ios.

One other point : @sanketmehta7 propose to encode headers in one unique string with the same encoding than the options argument : header1=value1,header2=value2.
but in this encoding values cannot include commas or equals characters.

an other solution is needed to included any characters from headers.

Start by reviewing the blocked #115 pull request and its Android-only approach, then inspect the plugin's Android and iOS request-header handling. Define a representation that preserves commas and equals characters in header values, and verify that headers are supported on both platforms.

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