Add getSchema() to SourceClient interface

[coded9]

Important Read

Addresses #77

What is the purpose of the pull request

This change adds getSchema() method to SourceClient interface

Brief change log

• Add getSchema() method to SourceClient
• Added unit tests for getSchema() in TestIcebergSourceClient

Verify this pull request

Run getSchema test in TestIcebergSourceClient

cc: @ashvina