Add platform dependency to Jackson BOM for Gradle consumers

[jjohannes]

See this mailing list discussion and this blog post.

This configures the 'gradle-module-metadata-maven-plugin' to write
and publish a Gradle Module Metadata file that contains a platform
dependency to the Jackson BOM.

All projects that have jackson-base as parent should then activate
this plugin an the use of Gradle Module Metadata by adding:

  <!-- do_not_remove: published-with-gradle-metadata -->

right below the <project> opening tag

As well as:

  <plugin>
    <groupId>de.jjohannes</groupId>
    <artifactId>gradle-module-metadata-maven-plugin</artifactId>
  </plugin>

to the <plugins> block.

[cowtowncoder]

Perfect! Only one more practical thing -- have I asked for CLA yet? If not (apologies if I have, sometimes my check fails), I'd need one from:

https://github.com/FasterXML/jackson/blob/master/contributor-agreement.pdf

(or matching corporate CLA, if preferable, most contributions are under individual one)
and it's only needed once for all contributions.
Usual method is to print, fill & sign, scan/photo, email to info at fasterxml dot com.
Once I get that I can merge these patches in.

[jjohannes]

@cowtowncoder I was wondering about CLA, but was not sure where to send it. Thanks for the explanation. I sent a mail.

[cowtowncoder]

Merged, published (mvn deploy) snapshot; manually merged to master as well.