Summary:
Follow-up to https://github.com/facebook/rocksdb/issues/8426 .

The patch adds a new kind of `InternalIterator` that wraps another one and
passes each key-value encountered to `BlobGarbageMeter` as inflow.
This iterator will be used as an input iterator for compactions when the input
SSTs reference blob files.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/8443

Test Plan: `make check`

Reviewed By: jay-zhuang

Differential Revision: D29311987

Pulled By: ltamasi

fbshipit-source-id: b4493b4c0c0c2e3c2ecc33c8969a5ef02de5d9d8