Add an implementation defined limit on number of keys in CSR

Rationale here is that many IRPC implementations are memory constrained.
We add a way for implementations to report the maximum number of
supported keys. This way we can guarantee consistent behavior across
different devices.

For implementation of IRPC version 3 and later we define the lowest
number of keys supported to be 20. This specific value was chosen
because the current implementation of RemoteProvisioner already combines
keys into batches of exactly 20.

Bug: 254137722
Test: atest VtsHalRemotelyProvisionedComponentTargetTest
Change-Id: Ib6fb6d6ec7c74004524a5505a37aa82c9e44ef91