Support object as value in extra_credential

[huw0]

Description

I want extra_credential to accept an object as a value. This is helpful because it allows pass through of short lived tokens (JWT or otherwise) that can be refreshed by the client without having to generate a new engine/client object.

Presently this is not possible because parse_quote does a check on the object type.

Therefore it is necessary to call str() on the extra_credential value during serialisation.

Non-technical explanation

Support object as value in extra_credential to support refreshing tokens.

Release notes

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* Support object as value in `extra_credential` to support refreshing tokens.