feat(proxy): enhance logging and deduplication for unknown datacenters

- Implemented a mechanism to log unknown datacenter indices with a distinct limit to avoid excessive logging.
- Introduced tests to ensure that logging is deduplicated per datacenter index and respects the distinct limit.
- Updated the fallback logic for datacenter resolution to prevent panics when only a single datacenter is available.

feat(proxy): add authentication probe throttling

- Added a pre-authentication probe throttling mechanism to limit the rate of invalid TLS and MTProto handshake attempts.
- Introduced a backoff strategy for repeated failures and ensured that successful handshakes reset the failure count.
- Implemented tests to validate the behavior of the authentication probe under various conditions.

fix(proxy): ensure proper flushing of masked writes

- Added a flush operation after writing initial data to the mask writer to ensure data integrity.

refactor(proxy): optimize desynchronization deduplication

- Replaced the Mutex-based deduplication structure with a DashMap for improved concurrency and performance.
- Implemented a bounded cache for deduplication to limit memory usage and prevent stale entries from persisting.

test(proxy): enhance security tests for middle relay and handshake

- Added comprehensive tests for the middle relay and handshake processes, including scenarios for deduplication and authentication probe behavior.
- Ensured that the tests cover edge cases and validate the expected behavior of the system under load.

src/config/types.rs

@@ -1156,6 +1156,13 @@ pub struct ServerConfig {
     #[serde(default = "default_proxy_protocol_header_timeout_ms")]
     pub proxy_protocol_header_timeout_ms: u64,
 
+    /// Trusted source CIDRs allowed to send incoming PROXY protocol headers.
+    ///
+    /// When non-empty, connections from addresses outside this allowlist are
+    /// rejected before `src_addr` is applied.
+    #[serde(default)]
+    pub proxy_protocol_trusted_cidrs: Vec<IpNetwork>,
+
     #[serde(default)]
     pub metrics_port: Option<u16>,
 
@@ -1180,6 +1187,7 @@ impl Default for ServerConfig {
             listen_tcp: None,
             proxy_protocol: false,
             proxy_protocol_header_timeout_ms: default_proxy_protocol_header_timeout_ms(),
+            proxy_protocol_trusted_cidrs: Vec::new(),
             metrics_port: None,
             metrics_whitelist: default_metrics_whitelist(),
             api: ApiConfig::default(),