support indexed fields proto

[nl5887]

Currently Ballista doesn't support indexed fields. This PR is needed for Ballista to support indexed fields.

[codecov-commenter]

Codecov Report

Merging #2707 (2eb7d8f) into master (1cd6ede) will decrease coverage by 0.02%.
The diff coverage is 0.00%.

❗ Current head 2eb7d8f differs from pull request most recent head 93b1d2a. Consider uploading reports for the commit 93b1d2a to get more accurate results

@@            Coverage Diff             @@
##           master    #2707      +/-   ##
==========================================
- Coverage   84.67%   84.64%   -0.03%     
==========================================
  Files         270      270              
  Lines       46919    46930      +11     
==========================================
- Hits        39727    39725       -2     
- Misses       7192     7205      +13     

Impacted Files	Coverage Δ
...physical-expr/src/expressions/get_indexed_field.rs	92.93% <0.00%> (-1.03%)	⬇️
datafusion/proto/src/from_proto.rs	33.79% <0.00%> (-0.32%)	⬇️
datafusion/proto/src/to_proto.rs	56.23% <0.00%> (-0.10%)	⬇️
datafusion/optimizer/src/utils.rs	31.72% <0.00%> (-0.54%)	⬇️
datafusion/expr/src/logical_plan/plan.rs	74.55% <0.00%> (-0.20%)	⬇️

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[andygrove]

LGTM. Thanks @nl5887