Add kotlin sources for maven-source-plugin

[rnimour]

Add maven-source-plugin and build-helper-maven-plugin to attach mixed java+kotlin sources, as this SO answer suggests.

I see that the source:jar command is called in a GitHub workflow, so explicitly putting the maven-source-plugin in the pom is not necessary. If I run ./mvnw source:jar locally, it will generate the sources including the kotlin files correctly, where the plugin configuration is inherited from oss-parent.

This should fix #171. In the first pull request to fix this, #175, I find that that author also included:

 <!-- explicitly define maven-deploy-plugin after other to force exec order -->
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-deploy-plugin</artifactId>
...

But I didn't see how this is necessary.

When this fix is verified, it can also be backported to 2.18 and 2.17.

Since I didn't make any code changes, do I still need to fill out the Contributor Licence Agreement?

[cowtowncoder]

@rnimour First of all: thank you for submitting this!

Any chance to redo this against 2.18? (or even 2.17) master is for upcoming 3.0.0, which is still months away, so it'd be good to get sources included in 2.x releases.

[cowtowncoder]

On CLA: not strictly needed since no shippable code changed (same goes for unit tests).

[rnimour]

Hi @cowtowncoder, alright sure, I've backported this change to the 2.18 and 2.17 branches, see #821 and #822

[cowtowncoder]

Thank you @rnimour ! Normally it'd be only necessary to do PR for the oldest branch and we can roll forward -- except for pom.xml changes where separate PRs are good (due to version conflicts, merge conflict resolution for pom.xml is "ignore theirs" so we won't be able to automatically get merges)

[cowtowncoder]

Thank you @rnimour ! I merged all PRs, verified they fix the issue. Should be good for 2.17.3 / 2.18.0(-rc1) releases.