Investigate caching exponentiation in IpaMultipoint

[thomas-quadratic]

Currently, there is no way of reusing exponentiation operations even when batching commitments.
However, it is possible to send related values together, triggering several identical exponential operations.

Investigate how a caching mechanism could work, whether the cache should live in rust or Java, and what modifications to Java should be done to take maximum advantage of a cache.
Also, there are configuration considerations concerning the potential cache size depending on the target deployment.