Paginated results when listing user access requests

[Wauplin]

Listing user access requests recurrently returns a list. However the backend API is soon to be introducing paginated results due to large number of requests for some repos resulting in memory/cpu peaks (see internal PR).

This PR updates the list_pending_access_requests, list_accepted_access_requests, and list_rejected_access_requests methods to return an iterator instead of a list. This allows us to lazy load requests from paginated results.

⚠️ Warning: this is a breaking change! Not great but hopefully these methods are not much used. Let's proactively contact teams we know are using it.

cc @Kakulukian @Pierrci

[HuggingFaceDocBuilderDev]

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[hanouticelina]

looks good! thanks

[Pierrci]

thanks!