Allow set up of settings.xml

[kwin]

Optionally a settings.xml should be set up with server settings for GPG and Maven deployment. This is necessary as the setup-java action will most likely no longer set up Maven (https://github.com/actions/setup-java/issues/60).

Look at https://github.com/actions/setup-java/blob/4003c04fbcd7c6d5d67fa11e57d706021663f6b2/src/auth.ts#L44 how this is done in setup-java currently and also at https://github.com/actions/setup-java/pull/63/files for the GPG support.

[rnc]

There is also https://github.com/s4u/maven-settings-action which works quite well as an alternative.

[slawekjaranowski]

There is https://github.com/marketplace/actions/setup-maven-action which can setup completly environment for maven also with settings.xml

[rnc]

@slawekjaranowski That is exactly the same as the one I mentioned.