feat: publish bt as cargo pkg to crates.io

[Cedric / ViaDézo1er (viadezo1er)]

Keep this PR to conserve the instructions to publish bt if we need it one day.

Requires

• the rust sdk to be updated on crates.io, I assume version `"0.1.0-alpha.3" but can be anything else.
• lingua to be published (as braintrust-lingua?)
  • braintrust-serde-json to be published. Due to Bug: f64 within flattened HashMap throws error on deserialization serde-rs/json#1157 and our fix https://github.com/braintrustdata/braintrust/pull/8779 we have duplicated serde-rs which we would need to publish too. It could be named braintrust-serde-json with a warning to use upstream serde instead since this is not for general use.
• run cargo update once this is done and these dependencies are added to crates.io

Would be nice to have the crate name bt, otherwise braintrust-bt?
As per rust-lang/crates.io#326 (comment) (comment from Tobias Bieniek, crates.io team co-lead), deleting [your] GitHub account will make your [crates.io] account inaccessible and your package unmaintained.
All of the repos/github pages/account listed on the maintainer page are 404. I believe that's why the maintainer could not transfer ownership to us (see emails in the sdk mailing list). See https://linear.app/braintrustdata/issue/SDK-38/packaging for details about this.
I suppose next step is contacting crates.io support.

The email of the (technically current) maintainer of the bt crate is in the cargo.toml: https://docs.rs/crate/bt/0.1.0/source/Cargo.toml, he is https://crates.io/users/ceram1.

CI doesn't pass since it's searching for the nonexistant crate/version.

[Luca Forstner (lforst)]

Obviously please wait for us to have the bt crate ownership before merging but lg