add feature for serde_bytes::ByteBuf

[ratmice]

Rebasing PR #252 on top of master, because the original PR wasn't made from a feature branch, it was easier to just open a new PR (and there was no substantial discussion lost).

Although it has been a while, I'm still interested in this patch for use in the linebender peniko crate.
Other linebender crates (kurbo) support schemars, but serde_bytes support is needed for peniko.
Supporting it there would currently allow some cleanups to CI in linebender/vello#733

serde_bytes being a dtolnay making it somewhat part of the serde ecosystem so at least from my perspective
this is a good argument for supporting the crate.

[ratmice]

I'll be honest I'm a little bit confused by the ci setup for this crate.
While I see many testsuite failures running e.g. cargo test,
I don't actually see the ones listed in the unsuccessful checks here when running locally.

[ratmice]

Probably going to take me a bit of time to go through all the changes in #340 to figure out what needs to be done to replace test_default_generated_schema.