Support for System Topic ($SYS)

[ssayebee]

I am trying to configure a dashboard using the $SYS/# topic.

Although there is support for special characters such as # and +, the $ character is still not supported, preventing me from subscribing to the $SYS/# topic.

mqtt-datasource/src/datasource.ts

Lines 13 to 14 in ef46f0a

	resolvedTopic = resolvedTopic.replace(/\#/gi, '__HASH__');
	const resolvedQuery: MqttQuery = {

I do not fully understand the source code, but I am aware that special character support for + and # is provided using the following trick:

+ -> __PLUS__
# -> __HASH__

I would like to request support for the $ character as well using __DOLLAR__.

Thank you for your efforts.