Change the logging levels on the fly

[pepyakin]

During debugging, it happened to me a couple of times that I wished I could change the logging levels without bringing down the whole node. In the case of polkadot-launch testing, that typically means bringing down the whole testnet.

It would be quite handy to have ability to change the logging levels on the fly. As a strawman idea, there could be a special flag that if enabled makes the node to expose an HTTP endpoint that listens on localhost for requests. Upon a POST request it will change the logging levels. The body format should be something simple so that it can be changed easily with curl.

[bkchr]

paritytech/substrate#7474

There is still something missing to make this work properly, but it should work in theory :D