Custom Domain Management

[KennethWussmann]

Add necessary endpoints to the client for custom domain management:

• [x] GET /api/custom_domains: Get custom domains.
• [ ] PATCH /api/custom_domains/:custom_domain_id: Update custom domain's information.
• [ ] GET /api/custom_domains/:custom_domain_id/trash: Get deleted aliases of a custom domain.

[KennethWussmann]

First one is already supported: https://github.com/KennethWussmann/simplelogin-client/blob/main/oas/paths/custom_domains/get.yml