[Customscript Account] Add importContract API

[altafan]

This API differs from getNextAddress by the fact that it doesn't perform a substitution of the namespace pubkey in the passed contract parameters + increment of the derivation index, like the latter does, but rather it discovers the derivation index of the one already present in the contract params object.

This method turns useful when the user wants to import an existing contract to the customscript account. Ideally he would retrieve the required template and the constructor params from a general contract metadata store. This service could be a block explorer that, similarly to etherscan, would support verification of contract by uploading template and constructor params and making them publicly accessible.