Add encrypt/decrypt throughput benchmarks

As an ironoxide user
I want to understand the encrypt/decrypt performance of the library

We have latency benchmarks for encrypt and decrypt, but not throughput ones. Some work was done on this on https://github.com/IronCoreLabs/ironoxide/tree/92-throughput but that branch is somewhat old now. It was not merged because we wanted to build the throughput benches around the async API, which did not exist at that time.

AC
* [ ] Unmanged encrypt throughput bench - operations/second
* [ ] Encrypt throughput bench - operations/second
* [ ] Unmanaged decrypt throughput bench - operations/second
* [ ] Decrypt throughput bench - operations/second
* [ ] (probably need this) a way to configure the tests to only process a certain number of requests per second (semaphore has been suggested for impl, but should think about this from first principles again)

Review the existing latency benchmarks and the old 92-throughput branch, then compare them with the current async API. Define benchmark coverage for managed and unmanaged encrypt/decrypt operations, including operations per second, and determine from first principles whether configurable request-rate limiting is needed; done means the listed throughput benchmarks run against the current library.

Written by the indexing model from the issue text.