(userd) Replace C++ userd with Rust implementation

- Add userd-rs crate with full user management functionality
- Remove C++ userd implementation (userd/ directory)
- Add userd binary to sonic-host-services-rs package
- Add userd.service to sonic-host-services-data package

Features:
- User CRUD operations (create, update, delete)
- Role-based access control (administrator, operator)
- SSH key management with validation
- Security policies and PAM faillock configuration
- Consistency checking on startup
- CONFIG_DB monitoring via SubscriberStateTable

The Rust implementation is a drop-in replacement for the C++ version,
producing the same 'userd' binary installed to /usr/local/bin/userd.

Signed-off-by: Manoharan Sundaramoorthy <manoharan@nexthop.ai>

Author: manoharan-nexthop

Cargo.lock

@@ -2,6 +2,7 @@
 resolver = '2'
 members = [
     "crates/procdockerstatsd-rs",
+    "crates/userd-rs",
 ]
 exclude = []
 
@@ -23,7 +24,7 @@ unused_import_braces = 'warn'
 [workspace.dependencies]
 # System info and SONiC database
 sysinfo = "0.35.0"
-swss-common = { git = "https://github.com/sonic-net/sonic-swss-common", branch = "master" }
+swss-common = { git = "https://github.com/sonic-net/sonic-swss-common", branch = "master", features = ["async"] }
 
 # Regex and pattern matching
 regex = "1.10"
@@ -50,4 +51,20 @@ syslog-tracing = "0.3"
 
 # Serialization
 serde = { version = "1.0", features = ["derive"] }
-serde_json = "1.0"
+serde_json = "1.0"
+
+# Error handling
+thiserror = "1.0"
+
+# Unix/POSIX system calls
+nix = { version = "0.29", features = ["user", "fs"] }
+
+# Async runtime and signal handling
+tokio = { version = "1", features = ["rt-multi-thread", "signal", "macros", "time"] }
+
+# Testing
+mockall = "0.13"
+tempfile = "3.10"
+
+# Static initialization
+lazy_static = "1.4"

Cargo.toml

@@ -0,0 +1,33 @@
+[package]
+name = "userd_rs"
+version.workspace = true
+edition.workspace = true
+description = "SONiC User Management Daemon in Rust"
+license.workspace = true
+authors.workspace = true
+
+[dependencies]
+swss-common.workspace = true
+serde.workspace = true
+serde_json.workspace = true
+tracing.workspace = true
+tracing-subscriber.workspace = true
+syslog-tracing.workspace = true
+libc.workspace = true
+thiserror.workspace = true
+nix.workspace = true
+tokio.workspace = true
+tempfile.workspace = true
+lazy_static.workspace = true
+
+[dev-dependencies]
+mockall.workspace = true
+
+[[bin]]
+name = "userd"
+path = "src/main.rs"
+
+[lib]
+name = "userd_rs"
+path = "src/lib.rs"
+