revid: move towards functional options approach for configuring revid pipeline

[Saxon Milton]

Revid is currently configured using a config struct, or map with variable value pairs, which is essentially representative of the config struct. We could make the revid API nicer and more allowing for future additions if we adopt a functional options approach to configuration, or at least have this as an option to start with. Eventually it would be nice to move here completely, but we’d have to write a shim between the variable-value approach (which is required by out cloud software) and the functional options methodology.