adds changes for `timestamp_offset` constraint

[desaikd]

Issue #9 #10

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

Grammar:

timestamp_offset: [ "[+|-]hh:mm"... ]

Ion Schema specification: https://amzn.github.io/ion-schema/docs/isl-1-0/spec#timestamp_offset

List of changes:

• adds changes for timestamp_offset constraint implementation
• adds a util implementation of TimestampOffset to create timestamp_offset constraint with valid timetsamp offset values
• adds TimestampOffsetLength enum variants for IslConstraint and Constraint
• adds TimestampOffsetConstraint implementations
• adds validation logic for timestamp_offset
• adds unit tests for timestamp_offset

Tests: added unit tests for timestamp_offset implementation.

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