test: benchmark feature toggle evaluation

[nunogois]

After discussing with @gardleopard we were interested in a naïve and simple benchmark for feature toggle evaluation in a language like Go.

This can be ran with a command like: go test -run=^$ -bench=BenchmarkFeatureToggleEvaluation -benchtime=10s

Here's an example of how the output could look like:

❯ go test -run=^$ -bench=BenchmarkFeatureToggleEvaluation -benchtime=10s
goos: darwin
goarch: arm64
pkg: github.com/Unleash/unleash-client-go/v3
BenchmarkFeatureToggleEvaluation-8 Final Estimated Operations Per Day: 101.131 billion (1.011315e+11)
13635154 854.3 ns/op
PASS
ok github.com/Unleash/unleash-client-go/v3 13.388s

The -8 in BenchmarkFeatureToggleEvaluation-8 stands for 8 cores (all the cores in my M1 CPU).
Clarification: This version of the code is actually only using 1 core, no parallelism.

I couldn't find a way of initializing our Unleash client without a listener, so I added a simple NoOpListener.

[gardleopard]

We tested this together and I love it! Would be good to add the how to run this in the README.md as well.

[nunogois]

We tested this together and I love it! Would be good to add the how to run this in the README.md as well.

Added a new mention to the benchmark in the README, let me know what you think. Addressed in b9dc624