feat: Add a way to define CORS origins using environment variables

[jubnl]

Useful for self host instances.

It will log out the CORS origins defined at startup.

Example:

.env:
CORS_ORIGINS=https://origin1.com,https://origin2.com
logs:

[2025-12-26T01:39:03Z INFO  vertd::http] CORS: allowed origins:
[2025-12-26T01:39:03Z INFO  vertd::http]   - https://origin1.com
[2025-12-26T01:39:03Z INFO  vertd::http]   - https://origin2.com

.env:
CORS_ORIGINS=*
logs:

[2025-12-26T01:54:45Z INFO  vertd::http] CORS: allow any origin (*)

.env with no CORS_ORIGINS key
logs:

[2025-12-26T01:55:55Z INFO  vertd::http] CORS: allow any origin (*)

CORS are enforced properly:
(running with CORS set to "example.com")