Add ANNSearcher class for easier ANN usage from code

[tteofili]

The changes involve creation of ANNSearcher class (similar to what SimpleSearcher does) for easier execution of approximate nearest neighbor search from code (e.g. from pyserini).
The PR also includes some minor changes to the original ANN code for eventually storing full vectors within the index (instead of requiring the text embedding model to be available at search time, as it is today).
Finally some minor adjustments to defaults for ANN analyzers, to make them consistent over the codebase.

[codecov]

Codecov Report

Merging #1078 into master will increase coverage by 0.34%.
The diff coverage is 88.23%.

@@             Coverage Diff              @@
##             master    #1078      +/-   ##
============================================
+ Coverage     45.00%   45.35%   +0.34%     
- Complexity      660      675      +15     
============================================
  Files           135      136       +1     
  Lines          8039     8092      +53     
  Branches       1149     1160      +11     
============================================
+ Hits           3618     3670      +52     
+ Misses         4098     4095       -3     
- Partials        323      327       +4     

Impacted Files	Coverage Δ	Complexity Δ
...a/io/anserini/ann/fw/FakeWordsEncoderAnalyzer.java	100.00% <ø> (+18.18%)	3.00 <0.00> (+1.00)
...ava/io/anserini/ann/lexlsh/LexicalLshAnalyzer.java	91.66% <ø> (ø)	5.00 <0.00> (ø)
...anserini/ann/ApproximateNearestNeighborSearch.java	75.00% <83.33%> (ø)	11.00 <0.00> (+4.00)
src/main/java/io/anserini/ann/IndexVectors.java	79.31% <83.33%> (-0.93%)	10.00 <0.00> (ø)
...anserini/search/SimpleNearestNeighborSearcher.java	94.44% <94.44%> (ø)	7.00 <7.00> (?)
...o/anserini/ann/ApproximateNearestNeighborEval.java	79.27% <100.00%> (ø)	13.00 <0.00> (ø)
...java/io/anserini/ltr/feature/CountBigramPairs.java	94.80% <0.00%> (+5.19%)	36.00% <0.00%> (+3.00%)

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[lintool]

@tteofili bunch of comments for you!