Split metadata tables into separate modules

[rshkv]

Split metadata tables into separate modules.

Context for this is to address #863 (comment) where the point was made that metadata_scan.rs will grow unwieldy if we shove all metadata table implementations in there. Especially as we're going to add extra utilities for those metadata tables.

The structure in this PR is:

inspect/
  metadata_table.rs: contains TableMetadata
  snapshots.rs: contains "snapshots" table
  manifests.rs: contains "manifests" table

In the future this can expand as described in #863 (comment).

[rshkv]

cc @liurenjie1024 @Xuanwo