fix(bindings): prevent temp connection free after panic

[jmayclin]

Description of changes:

We should never be freeing the temporary connection in with_context, even during an unwind. To prevent this make the connection ManuallyDrop during initialization.

Secondly, with_context may be invoked concurrently, so we should use context not context_mut.
Moved to #5701

Testing:

I added a unit test which confirms that the connection is not freed during a panic. I confirmed that the old behavior causes this test to fail.

~/workspace/s2n-tls/bindings/rust/extended$ cargo test panic_does_not_free_connection -- --nocapture

running 1 test
thread 'callbacks::tests::panic_does_not_free_connection' panicked at 'a panic in customer code', s2n-tls/src/callbacks.rs:121:13
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
thread 'callbacks::tests::panic_does_not_free_connection' panicked at 'assertion failed: `(left == right)`
  left: `1`,
 right: `2`', s2n-tls/src/callbacks.rs:141:9
error: test failed, to rerun pass '-p s2n-tls --lib'

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.