Add support for creating KeyValues from any iterable

[philwebb]

We're finding quite often that we need to adapt Iterable<Tag> to a KeyValues instance and writing code like this:

Iterable<Tag> tags = this.tagsProvider.getTags(context.getUriTemplate(), context.getCarrier(),
		context.getResponse());
KeyValues keyValues = KeyValues.empty();
for (Tag tag : tags) {
	keyValues = keyValues.and(tag.getKey(), tag.getValue());
}
return keyValues;

It seems like this pattern might be quite common (for example you might want to adapt a Map). It would be nice if it could be a oneliner:

return KeyValues.of(tags, Tag::getKey, Tag::getValue);

[philwebb]

If #3502 gets fixed before this then the test class I added will need to be rebased on top of it.

[sonatype-lift]

⚠️ 10 God Classes were detected by Lift in this project. Visit the Lift web console for more details.

[shakuzen]

Sounds reasonable to me. Thank you for the pull request.