db: shared sstable code and doc

[nerdroychan]

This PR contains the code of the implementation of shared (virtual) sstable.
The major updates are in the following files/directories:

─── pebble/
    ├── internal/
    │   └── manifest/
    │       ├── version.go
    │       └── version_edit.go
    ├── sstable/
    │   ├── reader.go
    │   ├── shared.go
    │   └── shared_test.go
    ├── compaction.go
    ├── disagg_test.go
    └── ingest.go

More implementation details can be found in sharedsst.md in the latest commit.

[cockroach-teamcity]

This change is