Add ACX5448 as a supported router

[aughera]

The ACX5448 is not currently supported by OpenJTS. I would love it to be as we are replacing all of our older ACX5048s with these!

HEAnet - my employer - has a testlab with an ACX5448 (and an ACX7024-X, but that is already supported) and I am happy to test with gnmic and OpenJTS as required.

There's a current bug with ACX5448s - they only do telemetry for virtual interfaces, not physical ones - but it's expected to be fixed in JunOS 25.1 according to the TAC.