Support Go execd plugins with shim (#7283)

plugins/inputs/execd/shim/README.md

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+# Telegraf Execd Go Shim
+
+The goal of this _shim_ is to make it trivial to extract an internal input plugin
+out to a stand-alone repo for the purpose of compiling it as a separate app and
+running it from the inputs.execd plugin.
+
+The execd-shim is still experimental and the interface may change in the future.
+Especially as the concept expands to prcoessors, aggregators, and outputs.
+
+## Steps to externalize a plugin
+
+1. Move the project to an external repo, optionally preserving the
+  _plugins/inputs/plugin_name_ folder structure. For an example of what this might
+  look at, take a look at [ssoroka/rand](https://github.com/ssoroka/rand) or
+  [danielnelson/telegraf-plugins](https://github.com/danielnelson/telegraf-plugins)
+1. Copy [main.go](./example/cmd/main.go) into your project under the cmd folder.
+  This will be the entrypoint to the plugin when run as a stand-alone program, and
+  it will call the shim code for you to make that happen.
+1. Edit the main.go file to import your plugin. Within Telegraf this would have
+  been done in an all.go file, but here we don't split the two apart, and the change
+  just goes in the top of main.go. If you skip this step, your plugin will do nothing.
+1. Optionally add a [plugin.conf](./example/cmd/plugin.conf) for configuration
+  specific to your plugin. Note that this config file **must be separate from the
+  rest of the config for Telegraf, and must not be in a shared directory where
+  Telegraf is expecting to load all configs**. If Telegraf reads this config file
+  it will not know which plugin it relates to.
+
+## Steps to build and run your plugin
+
+1. Build the cmd/main.go. For my rand project this looks like `go build -o rand cmd/main.go`
+1. Test out the binary if you haven't done this yet. eg `./rand -config plugin.conf`
+  Depending on your polling settings and whether you implemented a service plugin or
+  an input gathering plugin, you may see data right away, or you may have to hit enter
+  first, or wait for your poll duration to elapse, but the metrics will be written to
+  STDOUT. Ctrl-C to end your test.
+1. Configure Telegraf to call your new plugin binary. eg:
+
+```
+[[inputs.execd]]
+  command = ["/path/to/rand", "-config", "/path/to/plugin.conf"]
+  signal = "none"
+```
+
+## Congratulations!
+
+You've done it! Consider publishing your plugin to github and open a Pull Request
+back to the Telegraf repo letting us know about the availability of your
+[external plugin](https://github.com/influxdata/telegraf/blob/master/EXTERNAL_PLUGINS.md).