tor-keymgr: ephemeral type validation

Fixes #1917 (closed)

tor-keymgr:

• Updates the HasKind implementation for ArtiEphemeralKeystoreError to use an explicit match and adds a TODO.

• Adds item_type validation to Keystore::get and Keystore::remove for ArtiEphemeralKeystore, removing the TODO for #1917 (closed).

• Updates the tests for ArtiEphemeralKeystore to reflect the changes to Keystore::get and Keystore::remove.

/cc @gabi-250