Adds writers for SchemaDocument and types

[popematt]

Issue #, if available:

None

Description of changes:

• Changes IonSchemaWriter to be an object and the entry point to writing an Ion Schema.
• Creates an internal interface called VersionedIonSchemaWriter to replace the former IonSchemaWriter
• Adds ISL 2.0 writer implementations for Types and SchemaDocuments
• Creates a test harness that uses ion-schema-tests with the Ion Schema writer.

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

Attention: 18 lines in your changes are missing coverage. Please review.

Files	Coverage Δ
...lin/com/amazon/ionschema/writer/IonSchemaWriter.kt	100.00% <100.00%> (ø)
...n/ionschema/writer/internal/IonSchemaWriterV2_0.kt	71.42% <71.42%> (ø)
...amazon/ionschema/writer/internal/TypeWriterV2_0.kt	78.12% <78.12%> (ø)

... and 21 files with indirect coverage changes

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