Convert strings to binaries even if disabled or secret pending

[gomoripeti]

Proposed Changes

This way the type signature does not depend on the service state and is
consistently: decrypt(encrypt(Data)) -> binary().

Some users like rabbit_shovel_parameters rely on this
(https://github.com/rabbitmq/rabbitmq-server/blob/master/deps/rabbitmq_shovel/src/rabbit_shovel_parameters.erl#L447)

This should address crashes we've seen, when no secret was set yet and the shovel plugin crashed starting dynamic shovels. (Why there was no secret could be because of independent circumstances)

Alternatively it would be possible to support all terms unchanged (as since PR 15 encrypt_term is used under the hood). This would be a breaking change though.

Another alternative is to support unicode:chardata() instead of iodata() as input (but still always return binaries) which would more reflect the modern notion of a "string" - but probably more expensive to convert.

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