Add helper to get root invocation target from a zcap

[dlongley]

A helper that could be used to get the root invocation target from a zcap would be helpful because well-known API paths can be added onto the root invocation target -- for zcaps that grant authority to some base in the path hierarchy.

For example, if a zcap has a invocation target of either <root invocation target>, <root invocation target>/foo or <root invocation target>/foo/bar, each can be used to hit well-known path: <root invocation target>/foo/bar/baz. The helper would return <root invocation target> for any of these zcaps, allowing the target URL to then be constructed from that.

[dlongley]

Additionally, this code should be callable via a utility function:

https://github.com/digitalbazaar/ezcap/blob/v4.1.0/lib/ZcapClient.js#L309-L335

So two functions should be exposed, one to get the invocation target (even from a root zcap) and another to get the root invocation target.