[SDK][Breaking Changes] Add RequestClient with default Aptos Header + Refactor

[0xmigo]

Description

• Added RequestClient class. This act as a wrapper for UnityWebRequest
• Added default Aptos headers
• Refactor RestClient and FaucetClient to uses RequestClient

Test

Manually ran through SDK examples and make sure still works as expected

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[kPatch]

gm. Just did a merge of add_tokenclient to develop covering all the pending changes to the SDK. Fixing some merge conflicts then merging this branch. @banool @0xjinn

[0xmigo]

@kPatch Sounds good! Feel free to merge this PR whenever ready