Standardize Path resolution in XmlConfiguration

[joakime]

Jetty version
10.0.0

Java version
All

OS type/version
All

Description
Currently, due to the way we use <Set name="otherPath"><Property name="jetty.base" default="."/>/<Property name="jetty.other.path"/></Set> it's complicating how we can support arbitrary paths for the jetty.other.path property.

The introduction of new properties for absolute path and deep nesting to allow overrides/defaults/replacements is needlessly complicating the XML.

If we just used Path(base).resolve(configPath) we get support for all things (absolute, relative, windows, linux, server shares, etc. pretty much anything that Java and it's FileSystem can support, we can then support).