swh.core.api: Expose a serializable PagedResult object

This will allow typing paginated endpoints across our modules (swh.storage, swh.search, swh.indexer, ...).

As a first step, this will simplify swh-storage!987 (closed) which introduced it ;)

Related to T645

Test Plan

tox

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Migrated from D3632 (view on Phabricator)