gNMI depth extension

[Mrflatt]

Use Case

gMNI has few well-known extensions, which telegraf does not support yet.
Depth extension would be good first one, to allow controlling depth which response is parsed.

I already have made changes to use with our usecases, and will open pr, if well-known extensions should be supported by telegraf

Expected behavior

If depth value is added, []*gnmi_ext.Extension should be added to gnmi.SubscribeRequest.

[[inputs.gnmi]]
  ## Address and port of the gNMI GRPC server
  addresses = ["127.0.0.1:57777"]

  ## define credentials
  username = "cisco"
  password = "cisco"

  ## gRPC Depth Extension
  ## See https://github.com/openconfig/reference/blob/master/rpc/gnmi/gnmi-depth.md
  ## If specified, adds gNMI depth extension, which controls depth of the subtree to be returned in the response.
  depth = 2

Actual behavior

No depth option

Additional info

No response