gh-37329: add optional order= argument to .log() method for PARI finite-field elements
When the order of the base element of a finite-field logarithm is
already known, factoring the unit-group order can be skipped. PARI's
`fflog()` accepts the order of the base element as an optional input,
but that argument is currently not exposed in Sage. This patch takes
care of it. An example of the speedup is given in the new doctests.
URL: https://github.com/sagemath/sage/pull/37329
Reported by: Lorenz Panny
Reviewer(s): Giacomo Pope
