feat: Implement SSH Agent Connector and gRPC server

- Added `AgentConnectorTrait` and `AgentConnector` for managing agent connections.
- Introduced `SshAgentConnector` to handle SSH-related functionalities and start a gRPC server.
- Created database entities for `agents`, `certificates`, `organizations`, `public_key_revocations`, `setup_tokens`, `upstreams`, `users`, `virtual_hosts`, and `workspaces` using SeaORM.
- Developed `CertificateService` for managing certificate generation and retrieval.
- Implemented the main server logic to initialize the database connection and start the agent server.
- Configured development settings in `development.toml` for server and database connections.

apps/nxmesh-master/src/service/mod.rs

@@ -0,0 +1,40 @@
+use std::sync::Arc;
+
+use crate::{connector::agent::AgentConnectorTrait, service::certificate::CertificateService};
+
+pub mod agent;
+pub mod certificate;
+
+pub async fn start_master_server(
+    settings: crate::config::settings::Settings,
+    cli: crate::cli::Cli,
+) -> Result<(), Box<dyn std::error::Error + Send + Sync>> {
+    // Initialize database connection
+    let db_connection = crate::db::establish_connection(&settings.database.url).await?;
+
+    // Initialize certificate service with default cert folder path
+    let cert_service = Arc::new(crate::service::certificate::CertificateServiceImpl::new(
+        db_connection.clone(),
+        settings.grpc.certificate.cert_dir.clone(),
+        Arc::new(settings.clone()),
+    ));
+
+    // if generate_ca is set, generate a new certificate and exit
+    if cli.generate_ca {
+        // TODO: check the error type and return a more specific error message
+        cert_service.generate_ca_cert().await.ok();
+        println!("Certificate generated and stored successfully.");
+    }
+
+    // Initialize agent connector
+    let mut agent_connector = crate::connector::agent::AgentConnector::new(Box::new(
+        crate::connector::agent::ssh::SshAgentConnector::new(settings.clone())?,
+    ));
+
+    // Start the agent server
+    agent_connector
+        .start_server(&settings, cert_service, db_connection)
+        .await?;
+
+    Ok(())
+}