Make it possible and easy to manipulate next block's block_timestamp

[Nradko]

Some contracts must be tested in an e2e environment (using substrate-contract-node) with the possibility of testing their time-dependent behavior. To make such tests possible one should be able to manipulate the next block's block_timestamps. It would be great if such manipulation could be done using polkadot.js.
Currently, it is impossible to set a block_timestamp of the next block. The issue is to add such functionality