Added initial benchmarks

[czechboy0]

Motivation

To make it easier to add benchmarks for specific hot code paths and profile them.

Also to avoid regressing on allocation counts.

Modifications

Added a Benchmarks directory, similar to https://github.com/apple/swift-openapi-runtime/pull/119

Result

Now we have benchmarks.

Note that CI is coming later, and so are thresholds.

Test Plan

Ran the benchmarks locally.