feat(flux-dsl): allow to specify name of column in last function

[bednar]

Closes #215

Proposed Changes

Allow queries like:

Flux flux = Flux
    .from("telegraf")
    .last("_time");

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-io]

Codecov Report

Merging #216 (5288fae) into master (5be7b5f) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master     #216      +/-   ##
============================================
+ Coverage     88.39%   88.40%   +0.01%     
- Complexity      447      449       +2     
============================================
  Files           139      139              
  Lines          5516     5521       +5     
  Branches        227      227              
============================================
+ Hits           4876     4881       +5     
  Misses          561      561              
  Partials         79       79              

Impacted Files	Coverage Δ	Complexity Δ
...dsl/src/main/java/com/influxdb/query/dsl/Flux.java	96.12% <100.00%> (+0.02%)	144.00 <1.00> (+1.00)
...ava/com/influxdb/query/dsl/functions/LastFlux.java	100.00% <100.00%> (ø)	3.00 <1.00> (+1.00)

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