feat(mqttconn): add `pool_size` config parameter

That currently tunes the number of MQTT clients employed both for
subscriptions (if shared subscription is used) and for publishing to
a remote broker.

apps/emqx_connector/src/emqx_connector_mqtt.erl

@@ -177,6 +177,7 @@ mk_worker_opts(
     ResourceId,
     #{
         server := Server,
+        pool_size := PoolSize,
         proto_ver := ProtoVer,
         bridge_mode := BridgeMode,
         clean_start := CleanStart,
@@ -188,6 +189,7 @@ mk_worker_opts(
 ) ->
     Options = #{
         server => Server,
+        pool_size => PoolSize,
         %% 30s
         connect_timeout => 30,
         proto_ver => ProtoVer,

apps/emqx_connector/src/mqtt/emqx_connector_mqtt_schema.erl

@@ -73,6 +73,7 @@ fields("server_configs") ->
                 }
             )},
         {server, emqx_schema:servers_sc(#{desc => ?DESC("server")}, ?MQTT_HOST_OPTS)},
+        {pool_size, fun emqx_connector_schema_lib:pool_size/1},
         {clientid_prefix, mk(binary(), #{required => false, desc => ?DESC("clientid_prefix")})},
         {reconnect_interval, mk(string(), #{deprecated => {since, "v5.0.16"}})},
         {proto_ver,

apps/emqx_connector/src/mqtt/emqx_connector_mqtt_worker.erl

@@ -46,6 +46,7 @@
 -type options() :: #{
     % endpoint
     server := iodata(),
+    pool_size := pos_integer(),
     % emqtt client options
     proto_ver := v3 | v4 | v5,
     username := binary(),
@@ -66,6 +67,7 @@
 
 -type client_option() ::
     emqtt:option()
+    | {pool_size, pos_integer()}
     | {name, name()}
     | {ingress, ingress() | undefined}.
 
@@ -191,6 +193,7 @@ mk_client_options(Name, Ingress, BridgeOpts) ->
         end,
     Opts = maps:with(
         [
+            pool_size,
             proto_ver,
             username,
             password,