feat(external users): email sanity + mock SMTP

- SMTP has a mock to enable end to end test and validate the whole path
     (via OXICLOUD_SMTP_MOCK)
    - add email normalisation ( including punicode)
    - api to share to external user

Cargo.toml

@@ -70,6 +70,7 @@ lru = "0.16.4"
 fastcdc = "4.0.0"
 memmap2 = "0.9.10"
 lettre = { version = "0.11.18", default-features = false, features = ["smtp-transport", "tokio1-rustls-tls", "rustls-native-certs", "builder"] }
+idna = "1.1"
 
 [features]
 default = []