add get_current_mtp API call

[cipig]

We stumbled upon some electrum servers that set a wrong MTP which then leads to problems like #1281 in mm2.

It would be very helpful to have a get_current_mtp API call in mm2, so we can check the electrums for this problem and avoid those that have it and as such don't add them to ADEX Desktop/Mobile config.

[artemii235]

We stumbled upon some electrum servers that set a wrong MTP

@cipig Could you please provide more input on this? Basically, electrums do not set an MTP in any way, it's calculated in MM2 by requesting recent block headers and using their timestamps.

[shamardy]

@cipig This is ready for testing. @smk762 This is ready for documentation in the dev branch.

@borngraced could you please provide information about the new RPC method, please also add it to the postman collection and open a PR to developer-docs.

[borngraced]

The get_current_mtp RPC is used for getting the Median Time Past (MTP) of supported coins(UTXO).

— very useful for debugging some common bugs in mm2

USAGE

example request:

{
	"method": "get_current_mtp",
	"userpass": "USERPASS",
	"mmrpc": "2.0",
	"params": {
		"coin": "KMD"
	}
}

example response

{
   "mmrpc": "2.0",
   "result": {
   	"mtp": 1658746383
   },
   "id": null
}

[artemii235]

@cipig Did you have a chance to test it?

[smk762]

ref doc: https://developers.komodoplatform.com/basic-docs/atomicdex-api-20-dev/get_current_mtp.html

[cipig]

works fine, checked couple coins, compared to block explorer and the values look fine