feat: add configuration management with environment variable parsing

[fastrapier]

This pull request introduces a new configuration package for handling cluster provider settings via environment variables, and adds comprehensive unit tests to ensure correct behavior. It also updates dependencies to support the new implementation.

New configuration package and tests:

• Added pkg/config/config.go implementing a ClusterConfig struct with a ClusterProvider field, populated from the TEST_CLUSTER_PROVIDER environment variable using the caarlos0/env package. The New() function parses and validates this environment variable.
• Added pkg/config/config_test.go with unit tests for config.New, covering parsing the provider, handling missing or empty values, and multiple provider value cases.

Dependency updates:

• Added github.com/caarlos0/env/v11 and github.com/kelseyhightower/envconfig as indirect dependencies in go.mod to support environment variable parsing. [1] [2]

Documentation:

• Updated docs/WORKLOG.md to document the addition of unit tests for config.New and its coverage areas.