Parse provider IDs from season and episode folder/file names

Season and episode directories/files can now include provider ID
attributes in their names (e.g. "Season 01 [tvdbid=22222]" or
"Show S01E01 [tmdbid=99999].mkv"), consistent with the existing
behavior for series folders.

Supported providers: imdbid, tvdbid, tvmazeid, tmdbid.

Adds TmdbSeasonExternalId and TmdbEpisodeExternalId so that
the TMDB season and episode IDs are surfaced in the metadata editor.

Seasons do not have their own IMDb IDs, so we don't support imdbid parsing
in SeasonResolver. Instead, generate IMDb season URLs via
ImdbExternalUrlProvider using the parent series' IMDb ID and the
season number, matching the IMDb URL format:
imdb.com/title/{seriesId}/episodes/?season={N}

Add tests for the ExternalUrlProviders.

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