[discovery] Add ser2net mDNS USB serial discovery (#2519)

* Add ser2net mDNS USB serial discovery

* Add support for using multiple UsbSerialDiscovery services
* Add Ser2NetUsbSerialDiscovery that can use mDNS to discover ser2net RFC2217 serial ports
* Use discovered USB ports in SerialConfigOptionProvider

mDNS discovery is supported in ser2net 4.3.0 and newer.
E.g. you can install a ser2net version that provides this using APT in Ubuntu 21.04 and Debian 11.

Example ser2net YAML configuration that allows a serial port to be discovered using mDNS discovery:

%YAML 1.1
---
connection: &con01
  accepter: telnet(rfc2217),tcp,2222
  connector: serialdev,/dev/ttyUSB0
  options:
    mdns: true
    mdns-sysattrs: true
    mdns-name: devicename

Closes #1511

Signed-off-by: Wouter Born <github@maindrain.net>

bom/openhab-core/pom.xml

@@ -316,6 +316,12 @@
       <version>${project.version}</version>
       <scope>compile</scope>
     </dependency>
+    <dependency>
+      <groupId>org.openhab.core.bundles</groupId>
+      <artifactId>org.openhab.core.config.discovery.usbserial.ser2net</artifactId>
+      <version>${project.version}</version>
+      <scope>compile</scope>
+    </dependency>
     <dependency>
       <groupId>org.openhab.core.bundles</groupId>
       <artifactId>org.openhab.core.config.discovery.upnp</artifactId>