Ability to specify block sizes for magma

As a part of MB-54877,

• The /pools/default/buckets REST endpoint adds support for two new arguments:
  magmaKeyTreeDataBlockSize
  magmaSeqTreeDataBlockSize
• These are passed to kv in the bucket config as
  magma_key_tree_data_blocksize
  magma_seq_tree_data_blocksize
• These are supported on all magma configurations
• The arguments are not settable (POST) or gettable (GET) until the cluster is entirely on 7.2