Previously, Aether's configuration was largely hard-coded making it
impossible to configure a mirror, provide credentials for accessing
a repository, etc.

This commit adds support for configuring Aether via Maven's
settings.xml file. The support is optional and must be enabled by
grabbing spring-boot-maven-settings in an init script. The Aether
instance that's used when running the application will then be
configured using settings.xml. The settings file is expected to be
found in ${user.home}/.m2/settings.xml.

The configuration of the following items is currently supported:

 - Offline
 - Proxies
 - Mirrors
 - Server authentication
 - Local repository location

If the support is not enabled, settings.xml does not exist, or
settings.xml does not configure certain things then sensible defaults
are applied.