[Feature] Data-source and data property listener

[sczyh30]

Issue Description

Type: feature request

Describe what feature you want

Data-source and data property listener (SentinelProperty and PropertyListener in Java). Some considerations:

• We might need to support multiple SentinelProperty (with different parsers) shared by the single data-source.

[sczyh30]

As Golang lacks generic type at present, we might have to leave the value type as interface{} in SentinelProperty and PropertyListener :(

[sczyh30]

We may improve our design in the next version. Using channel to handle data propagating might be better (also in Golang style).

[louyuting]

I would add the general Property model in PR: #18

[louyuting]

We might use channel of golang to implement this feature.