Python bindings: expose various Ops through declarative builders

In particular, expose `cond_br`, `select` and `call` operations with syntax
similar to that of the previous emitter-based EDSC interface.  These are
provided for backwards-compatibility.  Ideally, we want them to be
Table-generated from the Op definitions when those definitions are declarative.

Additionally, expose the ability to construct any op given its canonical name,
which also exercises the construction of unregistered ops.

PiperOrigin-RevId: 238421583