feat(custom-domains): allow to register a custom domain (without UI) (#9879)

# In this PR
- Allow to register a custom domain
- Refacto subdomain generation

# In other PRs
- Add UI to deal with a custom domain
- Add logic to work with custom domain

packages/twenty-server/src/engine/core-modules/auth/dto/available-workspaces.output.ts

@@ -37,6 +37,9 @@ export class AvailableWorkspaceOutput {
   @Field(() => String)
   subdomain: string;
 
+  @Field(() => String, { nullable: true })
+  hostname?: string;
+
   @Field(() => String, { nullable: true })
   logo?: string;