Allows clients to specify the algorithm used via the `SignWithFlags` function. Currently this is `rsa-sha2-256` or `rsa-sha2-512` which are the two allowable options in the spec but it is extendable in the future as required.
Aiming to reduce code duplication, Existing `Sign` function retains it's default behaviour by calling `SignWithFlags` with `SignatureFlagReserved`.
The other `ExtendedAgent` function `Extension` returns `ErrExtensionUnsupported` to all calls as required by the spec.
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