Bug 1610: Add support for constructing ArtiEphemeralKeystore to InertTorClient::create_keymgr()

Resolves #1610, maybe #1106

I'm working on adding restricted discovery mode support to my arti-client tor provider in gosling. This patch (or something similar) is required to keep arti-client from creating+using a native keystore for keys.

I've verified cargo build and cargo test succeeds and I've updated gosling to use this functionality locally. No idea if this is the right approach but it seems to work, happy to revise to make it work better w/ your codebase o/