Add partition stats in snapshot summary

[jqin61]

Changes include:

• add write.summary.partition-limit to table properties
• add changed-partition-count to snapshot summary
• add partition stats. these are key value pairs whose key is like partitiion.<partition_key> and whose value is like a nested mini-summary string concatenated by ','.
  For example:
  'partitions.int_field=1': 'added-files-size=1234,removed-files-size=1234,added-data-files=1,deleted-data-files=1,added-records=100,deleted-records=100'

[jqin61]

@Fokko As we were discussing last time, it looks like the trustPartitionMetrics flag is always set to true in spark, so I ignored the flag. Please let me know the concerns if there is any!

[Fokko]

@Fokko As we were discussing last time, it looks like the trustPartitionMetrics flag is always set to true in spark, so I ignored the flag. Please let me know the concerns if there is any!

This is correct! It is only affected when we add manifests directly to the manifest list, but that's something we want to avoid (at almost all cost :)

[Fokko]

Small nits @jqin61 Looking good 👍

[Fokko]

This is great, thanks for working on this @jqin61