cargo_from_buck: add the patch section to Cargo workspace and allow workspace appending to existing manifest

Summary:
Two changes here:
1. The `[patch.crates-io]` section of `third-party/rust/Cargo.toml` is being now copied over to workspaces generated by autocargo for OSS and in the runtime generated Cargo.toml file for cargo-fbcode builds. Without that some projects could be buildable in Buck internally, but not externally on GitHub due to missing patches.
2. If a `[workspace]` Cargo.toml file is being generated and there is already a generated Cargo.toml file in the same directory then instead of overriding that file the `[workspace]` (and `[patch]`) sections are appended to that Cargo.toml file.

Reviewed By: farnz

Differential Revision: D22023144

fbshipit-source-id: dec54491c36c2ee0ab29eefb722b3eceaef6ffe1