feat(hooks): add onConfigDonloaded hook

[anativ]

Describe the purpose of your pull request

• Add new OnConfigDownloaded hook to notify when HTTP config download attempts are made
• The hook follows the same pattern as OnError by passing an error parameter directly (func(err error))
• Hook is called only for actual HTTP requests, not for cache reads or local-only operations
• nil error indicates successful download, non-nil error indicates failure

Related issues (only if applicable)

• N/A

How to test? (only if applicable)

• Affected component: Config fetching mechanism in config_fetcher.go
• Test coverage: Added table-driven test TestClient_Hooks_OnConfigDownloaded that verifies:
  • Hook is called on initial config download
  • Hook is called on subsequent refresh operations
  • Error parameter correctly reflects download success/failure
• Manual testing: Create a client with the hook configured and verify it's called during config downloads

Security (only if applicable)

• No security implications - this is a read-only notification hook that doesn't modify any data or expose sensitive information

Requirement checklist (only if applicable)

• I have covered the applied changes with automated tests.
• I have executed the full automated test set against my changes.
• [] I have validated my changes against all supported platform versions.
• I have read and accepted the contribution agreement.

This template highlights the key aspects of your change:
- **Purpose**: Clear explanation of the new hook functionality
- **Design**: Explains why it follows the `OnError` pattern
- **Scope**: Clarifies it's HTTP-only, not cache/local operations
- **Testing**: Shows you've added proper test coverage
- **Security**: Confirms no security risks

[adams85]

Thank you for the contribution!

Although I'm not the main maintainer of the Go SDK, I'd like to bring your attention to some broader considerations:

A similar hook already exists in the .NET and JavaScript SDKs. A few links regarding this:

• https://configcat.com/docs/sdk-reference/dotnet/#hooks
• https://github.com/configcat/.net-sdk/blob/master/src/ConfigCatClient/Hooks/IProvidesHooks.cs#L35

At a cursory glance, the onConfigDownloaded hook you suggest serves a pretty similar, if not the same, purpose.

If so, we should maintain consistency across SDKs as much as possible:

• We should use the same name as that of the existing hooks in other SDKs (i.e., onConfigFetched).
• The hook should have the same semantics. For example, the cache should be updated before calling the hook. (For example, see https://github.com/configcat/.net-sdk/blob/v9.4.3/src/ConfigCatClient/ConfigService/ConfigServiceBase.cs#L196)