Add translation plugins feature

[jeremydw]

Currently, translations can be imported from CSV files and Google Sheets natively within the translations UI. We should explore allowing project owners to add additional translation plugins, similar to the Google Sheets plugin, that allows translations to be imported/exported from other sources.

The general idea is we want to reduce the barrier to entry to acquiring translations and keep the integration as dead simple as possible.

We could do the following:

1. Add a new translations section to the cmsPlugin options
2. Add a field for each supported translation plugin, e.g. deepl, where the API key could be specified.
3. If configured, the new translation plugins would appear in the "Import" menu, e.g.

- Plugins
  -   Import from DeepL

Initially we could support DeepL, Gemini, and Crowdin.

Sample usage:

cmsPlugin({
  translations: {
    deepl: {
      apiKey: 'XXX',
    }
  }
  ...
})