adds changes for `annotations` constraint implementation

[desaikd]

Issue #9 #10

Description of changes: This PR works on adding implementation of annotations constraint.

Grammar:

<ANNOTATION> ::= <SYMBOL>
               | required::<SYMBOL>
               | optional::<SYMBOL>

<ANNOTATIONS_MODIFIER> ::= required::
                         | ordered::
                         | closed::

<ANNOTATIONS> ::= annotations: <ANNOTATIONS_MODIFIER>... [ <ANNOTATION>... ]

Ion Schema specification: https://amzn.github.io/ion-schema/docs/spec.html#annotations

List of changes:

• adds Annotations enum variants for IslConstraint and Constraint
• adds Annotation and AnnotationModifier implementations to be used to represent annotations
• adds AnnotationsConstraint implementation
• adds validation logic for annotations
• adds ViolationCode::AnnotationMismatched, ViolationCode::UnexpectedAnnotation, VioaltionCode::MissingAnnotation enum variant

Tests: added unit tests for annotations implementation.

• adds tests for programmatically creating annotations constraint
• adds tests for loading a schema using annotations constraint
• adds validation test for annotations constraint