[circle-opselector] Implement main logic

[mhs4670go]

What

Let's implement circle-opselector's main logic.

Why

There is a draft(#7600) that implements overall circle-opselector'. However, the implementer stopped the implementation for some reason. Moreover, it was a flatbuffers-aware(?) implementation, which a bit overlaps with the other module. We already have luci module that addresses conversion from flatbuffers contents to circle IR.

So, let's implement circle-opselector's main logic with this.

Related: #7596