Add an option <extraArgs> to let user pass custom parameters to protoc command

[sebastienvermeille]

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 👍