Summary:
This change introduces a new, lightweight _experimental_ API that reconstructs the [file # -> file checksum -> file checksum function] 1-1-1 mapping directly from the `MANIFEST` file considered `CURRENT` in scope of specific DB instance at the time. The goal is to provide a cheap alternative to `DB::GetLiveFilesMetaData` that doesn't require opening the database, reconstructing version sets and/or accessing files that are _potentially_ in disaggregated storage.
### Housekeeping:
1. Moved the `GetCurrentManifestPath` out of `version_set` to a new `manifest_ops` file(s) dedicated to manifest related operations.
2. Introduced new `Env::IOActivity::kReadManifest` to better reflect the IO intent in offline file checksum retrieving function.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/13178
Test Plan:
Added a unit test comparing the outcome of newly introduced API against the established `GetLiveFilesMetaData`:
```hcl
./db_test2 --gtest_filter="*GetFileChecksumsFromCurrentManifest_CRC32*"
```
Reviewed By: pdillinger
Differential Revision: D66711910
Pulled By: mszeszko-meta
fbshipit-source-id: 57091c550a14ac2e832bf7eea136dab5450e71bc
Maciej Szeszko