Make the the GitAttributesLineEndingsPolicy relocatable across machines

[nedtwigg]

...and remove FileSignature from it completely.

The easy way to understand it is this commit: 082f867, which introduces the new policy. The rest of the PR just removes the old one. The only downside is that now, when the line endings property is checked for equality (so on task up-to-dateness), the line-endings of every file in the target is computed and stored into this data structure:

spotless/lib-extra/src/main/java/com/diffplug/spotless/extra/GitAttributesLineEndings.java

Lines 109 to 114 in 758414b

	/** this is transient, to simulate PathSensitive.RELATIVE */
	transient final String rootDir;
	/** the line ending used for most files */
	final String defaultEnding;
	/** any exceptions to that default, in terms of relative path from rootDir */
	final ConcurrentRadixTree<String> hasNonDefaultEnding = new ConcurrentRadixTree<>(new DefaultCharSequenceNodeFactory());

The expensive part of this is iterating over a possibly large target. But we were doing this before anyway, and users can avoid it if they want by using LineEndings.UNIX, and taking on the git battle themselves.