[REQ] Allow configuring User-Agent in 'openapi-generator-maven-plugin'

Created by: MartinHatas

Allow configuring User-Agent in 'openapi-generator-maven-plugin' to override default User-Agent.

Example of usage:

<plugin>
    <groupId>org.openapitools</groupId>
    <artifactId>openapi-generator-maven-plugin</artifactId>
    <!-- RELEASE_VERSION -->
    <version>4.1.1</version>
    <!-- /RELEASE_VERSION -->
    <executions>
        <execution>
            <goals>
                <goal>generate</goal>
            </goals>
            <configuration>
                <inputSpec>${project.basedir}/src/main/resources/api.yaml</inputSpec>
                <generatorName>java</generatorName>
                <configOptions>
                   <sourceFolder>src/gen/java/main</sourceFolder>
                </configOptions>
                <httpUserAgent>${project.artifactId}/${project.version}</httpUserAgent>
            </configuration>
        </execution>
    </executions>
</plugin>