GIDINET: support init command

[TomOnTime]

Summary

Register CredsMetadata for GIDINET so the provider is offered by the dnscontrol init wizard.

Fields mirror the entries in integrationTest/profiles.json.

CC: @zupolgec

Test plan

• go build ./... passes
• dnscontrol init lists GIDINET as an option
• Verify any // TODO: Verify annotations in the diff (e.g. PortalURL)

🤖 Generated with Claude Code

[zupolgec]

Thanks @TomOnTime ! Builds clean, tests pass, and dnscontrol init lists GIDINET. One correction to the creds metadata, plus a related docs fix.

Creds metadata — see the inline suggestion. Gidinet doesn't authenticate with the control-panel login: the username is the normal account username, but the password must be the dedicated "API password" generated at https://www.gidinet.com/modules/private/account_password/. The suggestion resolves the // TODO: Verify on PortalURL (it's .com, and points right at the API-password page), adds a Notes line, and clarifies the field labels/help.

Docs — documentation/provider/gidinet.md isn't in this PR, but its Activation section currently claims the credentials "are the same ones you use to log in to the control panel," which is wrong for the password. Worth fixing in the same PR. Suggested replacement for that section:

## Activation

1. Log in to the [Gidinet Control Panel](https://www.gidinet.com/)
2. Generate an **API password** at [https://www.gidinet.com/modules/private/account_password/](https://www.gidinet.com/modules/private/account_password/)
3. In `creds.json`, use your account **username** together with that **API password** — the API password is distinct from your normal login password