Adds reader for timestamp_offset and timestamp_precision

[popematt]

Issue #, if available:

#256

Description of changes:

Adds readers for timestamp_offset and timestamp_precision; updates the ReaderTests accordingly.

Related PRs in ion-schema, ion-schema-tests, ion-schema-schemas:

None

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov]

Codecov Report

Patch coverage: 92.59% and project coverage change: +0.33 🎉

Comparison is base (e2f01e4) 82.50% compared to head (7ba8b8f) 82.84%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #266      +/-   ##
==========================================
+ Coverage   82.50%   82.84%   +0.33%     
==========================================
  Files         127      129       +2     
  Lines        3149     3176      +27     
  Branches      744      750       +6     
==========================================
+ Hits         2598     2631      +33     
+ Misses        327      322       -5     
+ Partials      224      223       -1     

Impacted Files	Coverage Δ
...lin/com/amazon/ionschema/reader/internal/ranges.kt	85.71% <81.81%> (+3.49%)	⬆️
...amazon/ionschema/reader/internal/TypeReaderV2_0.kt	92.85% <100.00%> (+0.21%)	⬆️
...ader/internal/constraints/TimestampOffsetReader.kt	100.00% <100.00%> (ø)
...r/internal/constraints/TimestampPrecisionReader.kt	100.00% <100.00%> (ø)

... and 3 files with indirect coverage changes

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