[Core] Owernership-based Object Directory - remove GCS-based object directory, add OBOD unit tests

[clarkzinzow]

We need to add unit tests for the ownership-based object directory. Note that the (current) GCS-based object directory doesn't have unit tests, and we could achieve testing coverage parity with the current object directory by giving an ownership-based object directory instance to the object manager in the object manager tests (and the stress tests).

Initial OBOD implementation was added in #9735.

[clarkzinzow]

The scope of this has been increased to include removing the GCS-based object directory.

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