Add type annotations to indexer classes

Classes and methods in swh/indexer/indexer.py, swh/indexer/metadata.py, swh/indexer/mimetype.py, swh/indexer/origin_head.py, swh/indexer/rehash.py currently do not have any type annotation. It would be nice to annotate all their functions.

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Migrated from T2258 (view on Phabricator)