[MNG-6392] Add Project Settings .mvn/settings.xml

[James Gao]

Hi.

Here is a proposal to improve the maven settings model in order to make
projects management easier. We need some opinion and discussion on it. If
everything is OK, we will work on it.


1. new settings merge order:

   - project level (${maven.multiModuleProjectDirectory}/.mvn/settings.xml)
   - user level
   - global



2. ignored fields in project level settings:

   - localRepository
   - interactiveMode
   - offline
   - usePluginRegistry


3. Do not need a new cli option to specify another project settings.