[inputs.gaussdb]Add new external input plugin

Hi everyone, I wrote an external input plugin for GaussDB, here's the project repository address,[**telegraf-input-gaussdb**](https://github.com/qixia1998/telegraf-input-gaussdb) please help me update it to the documentation. Thanks!
_https://github.com/qixia1998/telegraf-input-gaussdb_

Start by reviewing Telegraf's documentation for external input plugins and compare its organization and entry format with the linked telegraf-input-gaussdb repository. Add the GaussDB plugin information and repository link using the existing documentation conventions, then verify the documentation build or checks if available. Done means the external plugin is discoverable in the appropriate documentation section.

Written by the indexing model from the issue text.