Add client.AddTemplate benchmarks

[willbeason]

Add two benchmarks for compiling ConstraintTemplate: one simple template, and one complex template.

For the simple CT (and the general benchmarking code), I recommend looking at this code. See BenchmarkClient_AddTemplate and makeModule.

For the simple CT, we're trying to find the answer to the question of "What is the largest number of CTs a user could possibly have?"

For the complex CT, find a reasonably complex one from the gatekeeper-library. It doesn't particularly matter which one - so long as it looks complex. The idea is to answer the question "What is the largest number of normal CTs a user can have?"

It'd be best for this to live in its own test file - maybe addtemplate_benchmark_test.go?

[maxsmythe]

Here is the most complex CT I'm aware of:

https://github.com/open-policy-agent/gatekeeper-library/blob/master/library/general/containerlimits/template.yaml