Prototype implementation for private access poc (#8976)

## Problem
For the Private Access POC we want users to be able to disable access
from the public proxy. To limit the number of changes this can be done
by configuring an IP allowlist [ "255.255.255.255" ]. For the Private
Access proxy a new commandline flag allows to disable IP allowlist
completely.

See
https://www.notion.so/neondatabase/Neon-Private-Access-POC-Proposal-8f707754e1ab4190ad5709da7832f020?d=887495c15e884aa4973f973a8a0a582a#7ac6ec249b524a74adbeddc4b84b8f5f
for details about the POC.,

## Summary of changes
- Adding the commandline flag is_private_access_proxy=true will disable
IP allowlist

proxy/src/bin/local_proxy.rs

@@ -224,6 +224,7 @@ fn build_config(args: &LocalProxyCliArgs) -> anyhow::Result<&'static ProxyConfig
             rate_limiter_enabled: false,
             rate_limiter: BucketRateLimiter::new(vec![]),
             rate_limit_ip_subnet: 64,
+            ip_allowlist_check_enabled: true,
         },
         require_client_ip: false,
         handshake_timeout: Duration::from_secs(10),

proxy/src/bin/proxy.rs

@@ -224,6 +224,10 @@ struct ProxyCliArgs {
     /// Whether to retry the wake_compute request
     #[clap(long, default_value = config::RetryConfig::WAKE_COMPUTE_DEFAULT_VALUES)]
     wake_compute_retry: String,
+
+    /// Configure if this is a private access proxy for the POC: In that case the proxy will ignore the IP allowlist
+    #[clap(long, default_value_t = false, value_parser = clap::builder::BoolishValueParser::new(), action = clap::ArgAction::Set)]
+    is_private_access_proxy: bool,
 }
 
 #[derive(clap::Args, Clone, Copy, Debug)]
@@ -682,6 +686,7 @@ fn build_config(args: &ProxyCliArgs) -> anyhow::Result<&'static ProxyConfig> {
         rate_limiter_enabled: args.auth_rate_limit_enabled,
         rate_limiter: AuthRateLimiter::new(args.auth_rate_limit.clone()),
         rate_limit_ip_subnet: args.auth_rate_limit_ip_subnet,
+        ip_allowlist_check_enabled: !args.is_private_access_proxy,
     };
 
     let config = Box::leak(Box::new(ProxyConfig {