Benchmark Fetching Encryption Keys from Env

At present when encrypting a value before inserting it into the DB,
we are using the following code: `Application.fetch_env!(:fields, :encryption_keys)`
We need to benchmark this to determine if it has a significant performance impact.

Help is very much wanted/appreciated in designing this perf test.
Ideally we would run it in CI for a controlled CPU/RAM environment.

Ref: https://github.com/dwyl/fields/pull/83/files/1dccb1be01593939af8533f31ad79e62381400eb#r683231336