jmt: allow incremental tree overwrites for a given version

[erwanor]

Context:
Performing offline state migrations without incrementing the tree version: penumbra-zone/penumbra#3506.

This PR adds:

• TreeCache::new_overwrite gated behind a migration feature. This method instantiate a TreeCache against a root node key defined at the current version of the tree (rather than the previous one).
• JellyfishMerkleTree::append_value_set gated behind a migration feature. This method appends key/values against the current tree (using new_overwrite) and return the corresponding node batch.

We validated this approach in penumbra-zone/penumbra#4053

[conorsch]

Also performed implicit validation of behavior here, by testing a combination of penumbra-zone/penumbra#4053 & penumbra-zone/penumbra#4055.

[hdevalence]

This looks reasonable to me!