Develop own assemble to directly generate Circuit struct from QuantumCircuit

[hhorii]

What is the expected behavior?

Currently Qiskit-Aer uses QObj as the common representation of simulation targets.
However, as #1660 mentioned, recent QObj does not support latest features of QuantumCircuit.
Also, converting QObj is an apparent overhead because it is converted into Circuit struct of C++ in pybind.
By providing own assemble() to convert QuantumCircuit to Circuit struct of C++, we will get performance and flexibility for its specification.
This new feature will also resolve an issue of #1652.

[adekusar-drl]

@hhorii are there any plans when this enhancement can be implemented?

[hhorii]

@adekusar-drl this will be a new feature in 0.12.0, which will be released in middle of this February.

[hhorii]

#1717 was merged.