feat: add support for columns() function

[bednar]

Closes #336

Proposed Changes

Added support for columns() function.

Checklist

• CHANGELOG.md updated
• Rebased/mergeable
• A test has been added if appropriate
• mvn test completes successfully
• Commit messages are in semantic format
• Sign CLA (if not already signed)

[codecov-commenter]

Codecov Report

Merging #337 (9897244) into master (eaa88b3) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master     #337      +/-   ##
============================================
+ Coverage     88.86%   88.88%   +0.01%     
- Complexity      628      633       +5     
============================================
  Files           152      153       +1     
  Lines          6271     6279       +8     
  Branches        299      299              
============================================
+ Hits           5573     5581       +8     
  Misses          612      612              
  Partials         86       86              

Impacted Files	Coverage Δ
...dsl/src/main/java/com/influxdb/query/dsl/Flux.java	96.15% <100.00%> (+0.02%)	⬆️
.../com/influxdb/query/dsl/functions/ColumnsFlux.java	100.00% <100.00%> (ø)

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