Add an option to let user pass custom parameters to protoc command

That way we can use all available protoc plugins with their custom options

Applicable Issues
Fixes #61 #77

Description
Add configuration (optional) which is appended in the the protoc command invoked by the maven plugin.
This way we can easily address the issue of wanting to pass some specific parameters to protoc when running the plugin.

In my case I needed to pass some specific plugin arguments:

      <plugin>
        <groupId>org.xolstice.maven.plugins</groupId>
        <artifactId>protobuf-maven-plugin</artifactId>
        <executions>
          <execution>
            <id>protodoc</id>
            <goals>
              <goal>compile-custom</goal>
            </goals>
            <configuration>
              <pluginId>protoc-gen-doc</pluginId>
              <pluginExecutable>${project.build.directory}/protoc-plugins/protoc-gen-doc-1.4.1.linux-amd64.go1.15.2/protoc-gen-doc</pluginExecutable>
              <outputDirectory>${project.build.directory}/doc</outputDirectory>
              <extraArgs>--protoc-gen-doc_opt=markdown,README.md</extraArgs>
            </configuration>
          </execution>
        </executions>
        <configuration>
          <protocArtifact>com.google.protobuf:protoc:3.4.0:exe:${os.detected.classifier}</protocArtifact>
        </configuration>
      </plugin>

This way I can pass --protoc-gen-doc_opt=markdown,README.md and everything works fine.

Happy to be able to contribute to a such great project! I stay available for more assistance on it and to discuss it of course thank you 👍

Perform a release of version 2.1.0 including the changes of this external contribution #20

As maintainer doing this in my free time,
I strongly consider removing support of old versions of Java and benefit from new language features.
The same issue also occurs with the protoc protobuf versions used (they are not even available as download from github actions anymore (so old...)

The next release will probably be a major one including :

• full upgrade to JDK to 20
• full upgrade to latests protoc/protobuf versions
• full upgrade to latest maven version
• introduction of unit tests framework (easier and faster to test portion of code rather than the integrations tests already in place)

I leave this ticket open to give a bit time to think about it and listen to the community voice too if you have any reason to not upgrade etc.

A support for a very legacy java version (i.e. java 8) could be discussed and deployed if sponsored eventually. (As only companies are dealing with so outdated java version anyway I believe it's fair)

What do you think ?

For some reason the logs seems to be good but the release is not available on sonatype mirror servers.

I contacted the support and we are now following the new release 2.1.1 as they have already purged the 2.1.0 automatically. (Seems to be blocked in the staging area for some reason).

Let see

Document usage of the recently merged feature: #20
in the maven site deployed on gh-pages

maven site deployment via CI is failing

locally it pass

to be fixed

Sonar analysis of PRs is failing.

Should investigate a bit the reasons (probably not much to do)