feat(gatsby-source-contentful): add locale filter

[stoltzrobin]

Description

This allows the user to filter out locales that are not needed for the build. For example, if the space in Contentful is rather large with very many locales the javascript memory easiely runs out of memory.

Related Issues

#12938

[Khaledgarbaya]

The idea make sense LGTM

[pieh]

Is it possible to instruct contentful client to fetch data just for given locale? If not this seems good to go.

[pieh]

I think I just answered my own question when checking https://www.contentful.com/developers/docs/concepts/sync/

Because the Sync API retrieves all localized content, it might be better to use the delivery API to retrieve results of a single locale.

[stoltzrobin]

@pieh that was my initial though as well but as shown in the link that you posted it's not really the idea of the .sync()function. Maybe in the future it should be nice to support to pass queries in the sync call as well

[pieh]

Looking good now, gotta check it out in practice before merging

[stoltzrobin]

@pieh nice 👍

[thibautRe]

Hello @pieh, just checking if there's anything I can do to help get this approved and merged :)

[pieh]

Thanks @stoltzrobin, @Khaledgarbaya and @thibautRe for help with getting it merge-ready!