Make keymgr APIs less error prone by not using Box<dyn Any>

In !1337 (merged), we fixed a bug where read_ed25519_keypair would always return a type-erased () instead of a Ed25519Keypair due to a misplaced ;. Normally this sort of issue is prevented by type-checking, but in this case it wasn't because the return type of read_ed25519_keypair is (essentially) Box<dyn Any>.

We may be able to prevent this sort of error from happening again by changing the keymgr APIs to represent type-erased keys using a new StoredKey/ManagedKey trait instead of ErasedKey (i.e. Box<dyn Any>).