In planetside 2, what support activities give experience?

does a sunderer/S-AMS pilot get XP for providing a spawn point?

A deployed Sunderer will give the pilot 5 XP each time a player uses it as a spawn point.

does an engineer get XP for resupplying ammo?

For each tick of ammo resupplied to a unit, the engineer gains 10 XP.

under what circumstances does a combat medic (or engineer) gain xp for healing (or repairing)?

For every x seconds of continuous healing (or y hit points healed continuously, not sure) 10 XP is awarded to the medic. The same goes for repairs but with a reward of 5 XP. Reviving a player rewards 75 XP. Healing/repairing damage that was inflicted by an ally provides no XP.

In each of the above cases, the XP reward is increased when interacting with a player in your squad.