Introduce RSpec/AggregateExamples cop

Description of the proposal

It looks for opportunities to squash spec examples: https://www.rubydoc.info/gems/test-prof/0.11.2/RuboCop/Cop/RSpec/AggregateExamples

# bad
describe do
  specify do
    expect(number).to be_positive
    expect(number).to be_odd
  end

  it { is_expected.to be_prime }
end

# good
describe do
  specify do
    expect(number).to be_positive
    expect(number).to be_odd
    is_expected.to be_prime
  end
end

# fair - subject has side effects
describe do
  specify do
    expect(multiply_by(2)).to be_multiple_of(2)
  end

  specify do
    expect(multiply_by(3)).to be_multiple_of(3)
  end
end

Related discussion: https://gitlab.com/gitlab-org/plan/-/issues/145#note_411920773

The roll-out plan is to specify a file or directory to the Include section of the cop and auto-correct (or fix) the existing offenses: #250316