feat: Generate a parameter for the Accept header when a response code has multiple content types

[oliverkahrmann-basf]

This should fix #938

Adds an automatic parameter for the HTTP Accept header, if at least one of the response codes for an operation defines more than one content type.
This can then be used to decide the actual response:

func (h *ApiHandler) GetFileContent(ctx context.Context, params api.GetFileContentParams) (api.GetFileRes, error) {
	if params.Accept.MatchesContentType(api.MediaTypeApplicationVndOpenxmlformatsOfficedocumentSpreadsheetmlSheet) {
		// XLSX download
		// […] read/generate file
		buf *bytes.Buffer

		return &api.GetFileContentOKApplicationVndOpenxmlformatsOfficedocumentSpreadsheetmlSheet{
			Data: buf,
		}, nil

	} else {
		// JSON download
		// […] read/generate data

		res := &api.FileContent{
			// […] content
		}

		return res, nil

	}
}

[oliverkahrmann-basf]

This PR is ready for review.
Shall I squash the commits into one feat: … commit?

[ernado]

Shall I squash the commits into one feat: … commit?

One for code (feat: implement ...), one for generated files chore(gen): update.

Very sorry for late responce. Now iterations will be much quicker.