Teach the `rad sync` and `rad clone` commands to accept the
`--signed-refs-feature-level` option. This option configures that
fetch to use the minimum feature level provided when fetching from
other nodes. This overrides the value of
`node.fetch.signedReferences.featureLevel.minimum`, and should only be
used in scenarios where it is necessary to downgrade the security of a
fetch for backwards compatibility.
To achieve this, the `Handle::fetch` method introduces an optional
parameter for threading through the `FeatureLevel`.
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