Add to_thrift conversion for ColumnChunkMetaData and RowGroupMetaData

[sadikovi]

This PR adds to_thrift conversion for ColumnChunkMetaData and RowGroupMetaData, both structs already have from_thrift conversion.

I also added builders so metadata can be built outside of metadata.rs - this is used in writer when assembling metadata before converting it to Thrift.

Note that build() methods return Result, from_thrift() methods return Result, but to_thrift() methods return Self directly, because we ensure that metadata is built correctly.

Added a few tests to check the conversion.

[sadikovi]

@sunchao Could you review this PR as well? Thanks!

[coveralls]

Pull Request Test Coverage Report for Build 550

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at ?%

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Change from base Build 545:	0.0%
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💛 - Coveralls

[sadikovi]

@sunchao I updated the code according to your comments. Could you have a look again? Thanks!

[sadikovi]

I updated the code, could you review again? Let me know if there is anything missing!

[sunchao]

Merged. Thanks @sadikovi .

[sadikovi]

Thanks for merging, @sunchao!