Implement V2 registration support (#7765)

* Implement V2 registration support

* Require the client version header on every login, like upstream

Every user login (password, user API key, SSO, including a first SSO login)
now needs the Bitwarden-Client-Version header, for V1 accounts too. V2
accounts need 2026.10.1, or 2026.10.2 for the desktop app, upstream's
current minimum.

* Escape the email in the client version log line

On a first SSO login the email comes from the identity provider and isn't validated yet, so a newline in it could add a line to the log.

src/util.rs

@@ -899,6 +899,12 @@ pub fn parse_experimental_client_feature_flags(
         .collect()
 }
 
+/// Whether the admin enabled this supported client feature flag
+pub fn is_client_feature_flag_enabled(flag: &str) -> bool {
+    parse_experimental_client_feature_flags(&CONFIG.experimental_client_feature_flags(), &FeatureFlagFilter::ValidOnly)
+        .contains_key(flag)
+}
+
 /// TODO: This is extracted from IpAddr::is_global, which is unstable:
 /// https://doc.rust-lang.org/nightly/std/net/enum.IpAddr.html#method.is_global
 /// Remove once https://github.com/rust-lang/rust/issues/27709 is merged