chore: update ehttpc to 0.2.0

In ehttpc v0.2.0, we have optimized the implementation for request collection
to reduce resource consumption. see: emqx/ehttpc#31

mix.exs

@@ -49,7 +49,7 @@ defmodule EMQXUmbrella.MixProject do
       {:lc, github: "emqx/lc", tag: "0.2.1"},
       {:redbug, "2.0.7"},
       {:typerefl, github: "ieQu1/typerefl", tag: "0.9.1", override: true},
-      {:ehttpc, github: "emqx/ehttpc", tag: "0.1.12"},
+      {:ehttpc, github: "emqx/ehttpc", tag: "0.2.0"},
       {:gproc, github: "uwiger/gproc", tag: "0.8.0", override: true},
       {:jiffy, github: "emqx/jiffy", tag: "1.0.5", override: true},
       {:cowboy, github: "emqx/cowboy", tag: "2.9.0", override: true},

rebar.config

@@ -48,7 +48,7 @@
     , {redbug, "2.0.7"}
     , {gpb, "4.11.2"} %% gpb only used to build, but not for release, pin it here to avoid fetching a wrong version due to rebar plugins scattered in all the deps
     , {typerefl, {git, "https://github.com/ieQu1/typerefl", {tag, "0.9.1"}}}
-    , {ehttpc, {git, "https://github.com/emqx/ehttpc", {tag, "0.1.12"}}}
+    , {ehttpc, {git, "https://github.com/emqx/ehttpc", {tag, "0.2.0"}}}
     , {gproc, {git, "https://github.com/uwiger/gproc", {tag, "0.8.0"}}}
     , {jiffy, {git, "https://github.com/emqx/jiffy", {tag, "1.0.5"}}}
     , {cowboy, {git, "https://github.com/emqx/cowboy", {tag, "2.9.0"}}}