wip: Stub implementation of an OSS rust mysql_client library

[ben--]

WARNING: This is a WORK-IN-PROGRESS -- it enables the edenfs build to work but is likely non-functional

This PR includes a stub implementation of an OSS version of the internal-only mysql_client library.

Landing it now so that we can get the edenfs_linux GitHub action running again to prevent further bit-rot while OSS EdenFS comes online.

Tested through building Sapling/EdenFS (requires other fixes to run clean):

act --container-architecture linux/x86_64 -W .github/workflows/edenfs_linux.yml

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