feat: Add "OTHER" as a server provider

We should allow manual SelfPrivacy installations on unsupported server providers. The ServerProvider enum is one of the gatekeepers that prevent this and we can change it easily as not much server-side logic rely on this.

The next step would be manual DNS management, but it would be much more involved than just adding the enum value.

selfprivacy_api/graphql/queries/providers.py

@@ -14,6 +14,7 @@ class DnsProvider(Enum):
 class ServerProvider(Enum):
     HETZNER = "HETZNER"
     DIGITALOCEAN = "DIGITALOCEAN"
+    OTHER = "OTHER"
 
 
 @strawberry.enum