feat: add the ability to post a JSON payload to the Twilio API's

[RJPearson94]

Updating the configuration of a Flex instance requires a POST request to be made with a JSON payload. This change lays the foundations to support this. This is highlighted in https://github.com/twilio/twilio-oai-generator/issues/49

The PostJson accepts an interface for the request body (data) instead of just a map as this prevents the computational overhead of having to marshal the struct to a slice of bytes then create a string from the bytes. The JSON string would then be unmarshalled to a map and then marshalled again to a slice of bytes

This change makes the Content-Type header mandatory for POST requests

Checklist

• [x] I acknowledge that all my contributions will be made under the project's license
• [x] I have made a material change to the repo (functionality, testing, spelling, grammar)
• [x] I have read the Contribution Guidelines and my PR follows them
• [x] I have titled the PR appropriately
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• [x] I have added tests that prove my fix is effective or that my feature works
• [ ] I have added the necessary documentation about the functionality in the appropriate .md file
• [ ] I have added inline documentation to the code I modified

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