Support file metadata like Parquet

[hengfeiyang]

We are using parquet format and store custom metadata in file footer. but Vortex doesn't support it. can we think about this case. we need it.

[gatesn]

Yes I think this is totally reasonable.

The only things to think about are:

• We should probably store custom metadata in its own segment just before the footer. This allows it to use the same compression/encryption mechanisms when we add them later.
• how do we allow users to configure the location of this segment in the file. Could be at the front of the file. Or it could appear just before the footer. In the latter case, do we want to include it in the initial footer round trip, or do we want to wait for it to be explicitly loaded.