Added level of abstraction in indexers to use BaseIndexer for revisions

Summary:

• renaming methods filter_contents to filter and index_content to index

in all sub-classes and orchestrator

• renaming dependencies to ContentIndexer instead of BaseIndexer
• renaming in tests

Added RevisionMetadataIndexer with a detection tool for metadata

• RevisionMetadataIndexer takes a list of revisions and detects

in the root directory all the file names supported by the
swh-metadata-detector version 0.0.1 that can contain metadata

• checks if files where translated before in the content_metadata

table

• if not: sends the files to indexation

• aggregates results

Note: should keep results in revision_metadata but this part
is not ready in the storage

• also, changed init of ContentMetadataIndexer with tool in args

Updated documentation with new revision indexer

Test Plan: WIP (will be updated today)

Reviewers: ardumont

Differential Revision: https://forge.softwareheritage.org/D233