Adds readers for schema headers and footers

[popematt]

Issue #, if available:

#256

Description of changes:

• Adds readers for headers and footers, and tests.

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: 96.72% and project coverage change: +0.40 🎉

Comparison is base (f32fca9) 83.19% compared to head (00e1a88) 83.59%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #271      +/-   ##
==========================================
+ Coverage   83.19%   83.59%   +0.40%     
==========================================
  Files         131      133       +2     
  Lines        3201     3262      +61     
  Branches      756      768      +12     
==========================================
+ Hits         2663     2727      +64     
+ Misses        311      306       -5     
- Partials      227      229       +2     

Impacted Files	Coverage Δ
...m/amazon/ionschema/reader/internal/HeaderReader.kt	96.29% <96.29%> (ø)
...m/amazon/ionschema/reader/internal/FooterReader.kt	100.00% <100.00%> (ø)

... and 2 files with indirect coverage changes

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