[DX] warn when making POST proxy requests and using params

[Valahaar]

From Slack.

When using nango.post, the developer should be warned if they use params instead of data.

Furthermore, the data type annotation from ProxyConfiguration is unknown.
Is it possible to change it to something a bit more specific? (also, adding a few docs would be great, e.g., how is data treated by Nango? Is it expected to be an object that is encoded as JSON later? Or can we provide an already-stringified JSON?)