Add ability to override OSS Index server URL

[brianf]

Summary

This PR adds the ability for users to override the default OSS Index server URL with a custom URL. Previously, the URL was hardcoded to https://ossindex.sonatype.org with no convenient way to change it.

Changes

• Added SetOSSIndexURL() method to Configuration struct
• Added comprehensive test coverage for the new functionality
• Updated README with usage examples and documentation

Use Cases

This feature enables:

• Using custom OSS Index instances (e.g., enterprise deployments)
• Testing against local or staging environments
• Routing requests through proxies or gateways

Usage Example

configuration := ossindex.NewConfiguration()
configuration.SetOSSIndexURL("https://custom.ossindex.example.com")
apiClient := ossindex.NewAPIClient(configuration)

Test plan

• All existing tests pass
• New tests added for SetOSSIndexURL() functionality
• Tests cover default behavior, custom URLs, localhost, and IP addresses
• Backward compatibility maintained

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