GH-277: Support explicit plugin ordering

[ascopes]

• Add new feature that allows users to explicitly order plugins across multiple POMs by specifying <order>12345</order>.
• Document new behaviour.
• Document plugin common attributes in one place.
• Implement integration test for ordering.

Fixes GH-277.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.84%. Comparing base (d61e936) to head (0b7713a).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #278      +/-   ##
==========================================
+ Coverage   87.75%   87.84%   +0.09%     
==========================================
  Files          35       35              
  Lines        1004     1011       +7     
  Branches       73       73              
==========================================
+ Hits          881      888       +7     
  Misses         94       94              
  Partials       29       29              

Files	Coverage Δ
...bufmavenplugin/generation/SourceCodeGenerator.java	84.83% <100.00%> (+0.57%)	⬆️
...protobufmavenplugin/mojo/AbstractGenerateMojo.java	100.00% <ø> (ø)
...tobufmavenplugin/plugins/BinaryPluginResolver.java	71.19% <100.00%> (+0.50%)	⬆️
...protobufmavenplugin/plugins/JvmPluginResolver.java	78.95% <100.00%> (+0.38%)	⬆️
...opes/protobufmavenplugin/plugins/ProtocPlugin.java	100.00% <100.00%> (ø)