Small CRAM refactor: common ExternalEncoding Abstract Base Class

[jmthibault79]

Checklist

• Code compiles correctly
• New tests covering changes and new functionality
• All tests passing
• Extended the README / documentation, if necessary
• Is not backward compatible (breaks binary or source compatibility)

[jmthibault79]

The typed versions (Byte, Integer, etc.) actually all did the same thing because their encoding parameters are identical. The Codecs differ, but that's not relevant here.

[codecov-io]

Codecov Report

Merging #1346 into master will increase coverage by 0.29%.
The diff coverage is 100%.

@@              Coverage Diff               @@
##              master     #1346      +/-   ##
==============================================
+ Coverage     67.848%   68.139%   +0.29%     
- Complexity      8283      8448     +165     
==============================================
  Files            563       564       +1     
  Lines          33706     34302     +596     
  Branches        5657      5821     +164     
==============================================
+ Hits           22869     23373     +504     
- Misses          8659      8732      +73     
- Partials        2178      2197      +19

Impacted Files	Coverage Δ	Complexity Δ
...s/cram/encoding/external/ExternalLongEncoding.java	57.143% <100%> (-9.524%)	2 <1> (-1)
...tools/cram/encoding/external/ExternalEncoding.java	100% <100%> (ø)	2 <2> (?)
...s/cram/encoding/external/ExternalByteEncoding.java	100% <100%> (ø)	5 <1> (-1)	⬇️
.../samtools/cram/build/CompressionHeaderFactory.java	89.734% <100%> (-0.376%)	64 <0> (ø)
...ram/encoding/external/ExternalIntegerEncoding.java	100% <100%> (ø)	5 <1> (-1)	⬇️
...n/java/htsjdk/samtools/cram/structure/SliceIO.java	84.524% <0%> (+1.19%)	15% <0%> (+1%)	⬆️
...c/main/java/htsjdk/samtools/util/IntervalList.java	76.321% <0%> (+2.424%)	131% <0%> (+62%)	⬆️
...java/htsjdk/samtools/cram/structure/Container.java	91.781% <0%> (+3.737%)	31% <0%> (+4%)	⬆️
...samtools/util/AsyncBlockCompressedInputStream.java	76% <0%> (+4%)	13% <0%> (+1%)	⬆️
... and 4 more