perf(): filter by path when loading collection from github backend

[demshy]

Summary

When using the beta "path" option on collections, Github backend was loading all entries in the folder and then filtering them on frontend. On a large use case (4-5 collections in the same parent folder with a lot of entries) this starts to affect performance especially when requests are not cached yet.
This PR adds a filter that will consider the path and i18n language (if this is enabled) before requesting files from backend, therefore significantly decrease the number of xhr requests.
We have been using this on our fork in a large production app for a year now and found no issues so far.

Test plan
To test this, you need to set up collections separated by path, connect straight to the Github backend and observe the number of requests between master branch and this.

collections:
  - name: properties
    ...
    folder: content/properties
    path: '{{slug}}/_index'
    ...
  - name: property-units
    folder: content/properties
    path: '{{slug}}/{{unitlist}}'

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