feat: enable reuse of FromData instance

[GLGDLY]

I have been facing a situation that an API I was requesting would have some retryable error especially with form-data requests. While with reusing the FormData instance in my request, the RuntimeError was raised. In this case, I was confused that why the FormData instance would raise error upon re-gen.

With looking into the source code of aiohttp, I cannot discover any possible errors that may occurs with the reuse of FormData instance, especially the MultipartWriter seems to be designed with the ability to write() for more than once. As a result, I tried the change in this pr in my own code to test and proved that the resulting form-data to be send is correctly outputted.

To work more, with the fact that MultipartWriter seems to be designed for more than one time usage, the FormData may also accept further adding fields into the writer after already generated form data for a certain number of times. With this, it might be more flexible for we to deal with form-data request.

What do these changes do?

• do not raise RuntimeError with the reuse of FormData instance but returning self._writer
• after already generated form data for a certain number of times, allow adding more fields to further-update the MultipartWriter instance

Are there changes in behavior for the user?

Users may reuse the FormData instance without the need of creating a new instance for every form-data requests.

Related issue number

n/a

Checklist

• [x] I think the code is well written
• [x] Unit tests for the changes exist
• [x] Documentation reflects the changes
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