[MNG-5659] Provide project-specific settings located in ${session.rootdir}/.mvn/settings.xml by default

[gnodet]

This add support for project specific settings using:

• a default location of ${session.rootDirectory}/.mvn/settings.xml
• an alternate location specified by -ps or --project-settings and resolved against the current dir if provided with a relative path

Note that project settings have some limitations:

• no localRepository
• no interactiveMode
• no offline
• no proxies
• no server[x].username, server[x].password, server[x].privateKey, server[x].passphrase, server[x].filePermissions, server[x].directoryPermissions
• no usePluginRegistry

Simple IT provided with apache/maven-integration-testing#265

===
Related discussions to be considered (but note that this PR does not depend on the following):

• MNG-6762 suggests to also modify other file resolution, but there was a strong opposition to modify the existing behaviour. This PR is coherent with the current code where relative files are resolved against the current directory.

[gzm55]

usePluginRegistry in project settings should also be ignored.

[rmannibucau]

Can ${maven.multiModuleProjectDirectory}/.mvn/settings.xml become ${maven.multiModuleProjectDirectory}/.mvn/project.settings.xml or something like that since .mvn/settings.xml is sometimes used by users so something more unlikely would be great.

[cstamas]

While I like this, I'd generalize the approach (maybe no now) in similar way like "chained LRM" works. Example:

-s remains as today "user settings override"
And "settings tail" is s1, s2, s3.... global setting (that also can be overridden from CLI). And this list is ordered from "most dominant" to "most recessive", meaning, that even settings builder could be modified to make a for-loop to build effective settings in this order:
"user settings", s1, s2, s3.... "global settings"