feat(getFeatureVariable): Add getFeatureVariable API

[yasirfolio3]

Summary

• Add GetFeatureVariableBoolean as a function in client.go
• Add GetFeatureVariableDouble as a function in client.go
• Add GetFeatureVariableInteger as a function in client.go
• Add GetFeatureVariableString as a function in client.go
• Add GetFeatureFlagByKey() function in datafileprojectconfig/config.go
• Add GetVariable() function in FeatureFlag entity
• Add MapFeatureFlags() function in datafileprojectconfig/mappers/feature.go

Tests

• Unit tests in client_test.go

Issues

OASIS-5006

[mikeproeng37]

Please break up the PR a little by first introducing GetFeatureVariableString. Once we approve and merge that in then we'll add the rest of em. It just makes reviewing easier.

[yasirfolio3]

Please break up the PR a little by first introducing GetFeatureVariableString. Once we approve and merge that in then we'll add the rest of em. It just makes reviewing easier.

done, closing this PR for now, please review #71 .