ARROW-11822: [Rust][Datafusion] Support case sensitive comparisons for functions and aggregates

[alamb]

Broken out from #9600 by @wqc200. Note this does not contain the part of #9600 that controls the output display of functions.

Rationale

Aggregate functions are checked using case insensitive comparison (e.g. select MAX(x) and select max(x) both work - the code is here

However, scalar functions, user defined aggregates, and user defined functions, are checked using case sensitive comparisons (e.g. select sqrt(x) works while select SQRT does not. Postgres always uses case insensitive comparison:

alamb=# select sqrt(x) from foo;
sqrt
------
(0 rows)
alamb=# select SQRT(x) from foo;
sqrt
------
(0 rows)

Changes

Always use case insensitive comparisons for unquoted identifier comparison, both for consistency within DataFusion as well as consistency with Postgres (and the SQL standard)

Adds tests that demonstrate the behavior

Notes

This PR changes how user defined functions are resolved in SQL queries. If a user registers two functions with names
"my_sqrt" and "MY_SQRT" previously they could both be called
individually. After this PR my_sqrt will be called unless the user
specifically put "SQRT" (in quotes) in their query.

[github-actions]

https://issues.apache.org/jira/browse/ARROW-11822

[codecov-io]

Codecov Report

Merging #9827 (1440a98) into master (05a507c) will increase coverage by 0.01%.
The diff coverage is 98.63%.

@@ Coverage Diff @@## master #9827 +/- ##
==========================================
+ Coverage 82.42% 82.43% +0.01% 
==========================================
Files 252 252 Lines 58977 59043 +66 ==========================================
+ Hits 48609 48674 +65 - Misses 10368 10369 +1 

Impacted Files	Coverage Δ
rust/datafusion/src/sql/planner.rs	83.55% <83.33%> (+0.07%)	⬆️
rust/datafusion/src/execution/context.rs	91.43% <100.00%> (+0.56%)	⬆️
rust/datafusion/src/physical_plan/aggregates.rs	90.90% <100.00%> (ø)
rust/parquet/src/encodings/encoding.rs	94.86% <0.00%> (-0.20%)	⬇️

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[alamb]

fyi @wqc200@Dandandan@seddonm1 and @returnString

[jorgecarleitao]

LGTM. this beauty in just less than 20 LOC. 💯