Lock Engines' Git Dependencies to Specific Commits

[Brendonovich]

Problem

Engine dependencies like quaint and mysql_async are currently referred to by branch names, but this results in breaking changes being propagated to Prisma Client Rust even though I already lock to specific commits of the engines. Recently PCR's MySQL support has broken because mysql_async received a breaking update - quaint depends on mysql_async via branch, so when Cargo resolves quaint's dependencies it just uses the latest - and incomaptible - version of mysql_async.

Suggested solution

Use rev = "..." instead of branch = "..." when specifying git dependencies in the engines.

Alternatives

I just keep forking and maintaining stable versions of each engine dependency.

Additional context

First discussed in #collab-custom-prisma-clients in Slack.
Would fix Brendonovich/prisma-client-rust#256