Speed up policy lookups in DeclarativePolicy

What does this MR do and why?

Add a global cache to cache subject's mapping to their policy classes.

This patches will be upstreamed to declarative-policy gem soonish.

See gitlab-org/ruby/gems/declarative-policy#30 and !119924 (merged).

How to set up and validate locally

Heavily inspired by !119924 (merged).

Benchmark code

# frozen_string_literal: true

require "./config/environment"

Rails.configuration.eager_load_namespaces.each(&:eager_load!)
puts "Application is loaded."

require 'benchmark/ips'

class Project2 < Project
  def self.declarative_policy_class
    :ProjectPolicy
  end
end

user = User.first
subject_1 = Project.first
subject_2 = Project2.first

Benchmark.ips do |b|
  b.report('implicit') do
    DeclarativePolicy.policy_for(user, subject_1)
  end

  b.report('explicit') do
    DeclarativePolicy.policy_for(user, subject_2)
  end

  b.compare!
end

Before

Application is loaded.
Warming up --------------------------------------
            implicit   463.000  i/100ms
            explicit    17.589k i/100ms
Calculating -------------------------------------
            implicit      4.232k (±27.7%) i/s -     18.520k in   5.073017s
            explicit    163.059k (± 6.6%) i/s -    826.683k in   5.092063s

Comparison:
            explicit:   163058.5 i/s
            implicit:     4232.2 i/s - 38.53x  slower

After

Application is loaded.
Warming up --------------------------------------
            implicit    16.562k i/100ms
            explicit    15.042k i/100ms
Calculating -------------------------------------
            implicit    139.730k (± 8.8%) i/s -    695.604k in   5.020544s
            explicit    124.751k (± 8.7%) i/s -    631.764k in   5.100487s

Comparison:
            implicit:   139730.3 i/s
            explicit:   124750.5 i/s - same-ish: difference falls within error

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.