[DISSCUS]: support `ctx.var` to get service_name and route_name

[Firstsawyou]

Issue description

When using the http-logger plugin, I need to obtain the name value corresponding to the service (for example: "service_name:$service_name"). But it is currently not available.
Is it necessary for us to support ctx.var to get the value of service_name?
By the way, sometimes route_name is also obtained, but it is not currently supported.

Environment

• apisix version (cmd: apisix version): 2.0
• OS:

[membphis]

I think this feature is useful, look nice for me

[Firstsawyou]

Assigned to me.