Make MQTT optional but default

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "svc-agent"
-version = "0.21.0"
+version = "0.22.0"
 authors = ["Andrei Nesterov <ae.nesterov@gmail.com>"]
 description = "An agent library."
 readme = "README.md"
@@ -14,7 +14,9 @@ edition = "2018"
 doctest = false
 
 [features]
+default = ["mqtt"]
 queue-counter = []
+mqtt = ["rumqttc"]
 sqlx = ["dep:sqlx", "svc-authn/sqlx"]
 
 [dependencies]
@@ -23,7 +25,7 @@ base64 = "0.21"
 chrono = { version = "0.4", features = ["serde"] }
 http = "0.2"
 log = "0.4"
-rumqttc = "0.7"
+rumqttc = { version = "0.7", optional = true }
 serde = { version = "1.0", features = ["derive" ] }
 serde_json = "1.0"
 sqlx = { version = "0.6", features = ["runtime-tokio-native-tls", "postgres"], optional = true }