Add readers for annotations and contains constraints

[popematt]

Issue #, if available:

#256

Description of changes:

Adds readers for Annotations (v2) and Contains constraints. Updates ReaderTests.

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: 96.29% and project coverage change: +0.45 🎉

Comparison is base (b774d33) 82.84% compared to head (7d82c40) 83.29%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #267      +/-   ##
==========================================
+ Coverage   82.84%   83.29%   +0.45%     
==========================================
  Files         129      131       +2     
  Lines        3176     3203      +27     
  Branches      750      756       +6     
==========================================
+ Hits         2631     2668      +37     
+ Misses        322      310      -12     
- Partials      223      225       +2     

Impacted Files	Coverage Δ
...reader/internal/constraints/AnnotationsV2Reader.kt	94.73% <94.73%> (ø)
...amazon/ionschema/reader/internal/TypeReaderV2_0.kt	90.27% <100.00%> (-2.58%)	⬇️
...hema/reader/internal/constraints/ContainsReader.kt	100.00% <100.00%> (ø)

... and 4 files with indirect coverage changes

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