Update OAuth instructions to use curl to get refresh token

[thena-seer-sfg]

Summary

The current instructions are incorrect. The code received in the redirect url is not a refresh token but a short-term authorization code, and the tap fails when it tries to use it as one. In order to get a refresh token, there is a code exchange step. The easiest way to do this is using curl, so I've added this step to the instructions.

This is based on Harvest's documentation: https://help.getharvest.com/api-v2/authentication-api/authentication/authentication/#oauth2-authorization-flow

I have also opened this PR in the source repo, where it seems these instructions were copied from: https://github.com/singer-io/tap-harvest-forecast/pull/28

Test plan

Follow the updated instructions when setting up the extractor in meltano. Verify meltano config tap-harvest-forecast test confirms the validity of the configuration.

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