internal/manifest: consider removing Trailer from file boundary keys

Currently, file boundaries are represented using `InternalKey`s, encoding user keys along with sequence number and key kind. Now that user keys are never split between files of a compaction and all existing instances of split user keys have been compacted (since [FormatSplitUserKeysMarkedCompacted](https://github.com/cockroachdb/pebble/blob/8dc60b62ebbfc9295e16e1910d4646b27a4a0770/format_major_version.go#L88-L95) / Cockroach v22.2), the internal key type is unnecessary. We could represent the file boundaries using a simpler type, holding a tuple of (user key, is exclusive).

Jira issue: PEBBLE-174