[assemble_maven] add developers tag support for pom (to support Maven Central)

[sugarmanz]

What is the goal of this PR?

Support publishing to Maven Central again. Was originally done in #133, but the developers tag was never migrated to the maven rule rewrite in #296.

What are the changes implemented in this PR?

Add an optional developers attribute to assemble_maven rule as a string_list_dict. Developers can be described as such:

developers = {
  "sugarmanz": ["name=Jeremiah Zucker", "[email protected]"],
  "adierkens": ["name=Adam Dierkens"],
}

And will be populated as:

<developers>
  <developer>
    <id>adierkens</id>
    <name>Adam Dierkens</name>
  </developer>
  <developer>
    <id>sugarmanz</id>
    <name>Jeremiah Zucker</name>
    <email>[email protected]</email>
  </developer>
</developers>