SK Model QAOA application benchmark

[mpharrigan]

• SKModelQAOASpec and sk_model_qaoa_spec_to_exe wraps existing QAOA code into the cirq_google.workflow tools so this can serve as an algorithmic benchmark.
• scripts and recirq.algorithmic_benchmark_library entry to follow
• Small changes to the circuit generation code to support generating lightly-compiled executables so the benchmark can target multiple gatesets following https://github.com/quantumlib/Cirq/pull/5336