Add `parseGenericOperation()` to the OpAsmParser

This method parses an operation in its generic form, from the current parser
state. This is the symmetric of OpAsmPrinter::printGenericOp(). An immediate
use case is illustrated in the test dialect, where an operation wraps another
one in its region and makes use of a single-line pretty-print form.

PiperOrigin-RevId: 267930869