fix: add python stdio implement

[whybeyoung]

fix: #143 and #157

[hashicorp-cla]

All committers have signed the CLA.

[SSSOCPaulCote]

@berlinsaint I'm running into the same Method not found! with your fix.

$ go build -o kv
export KV_PLUGIN="python plugin-python/plugin.py"
./kv put hello world
2022-09-08T04:41:00.394Z [DEBUG] plugin: starting plugin: path=/bin/sh args=[sh, -c, "python plugin-python/plugin.py"]
2022-09-08T04:41:00.399Z [DEBUG] plugin: plugin started: path=/bin/sh pid=21908
2022-09-08T04:41:00.402Z [DEBUG] plugin: waiting for RPC address: path=/bin/sh
2022-09-08T04:41:00.564Z [DEBUG] plugin: using plugin: version=1
2022-09-08T04:41:00.567Z [TRACE] plugin.stdio: waiting for stdio data
2022-09-08T04:41:00.569Z [DEBUG] plugin.stdio: received EOF, stopping recv loop: err="rpc error: code = Unimplemented desc = Method not found!"
Error: rpc error: code = Unimplemented desc = Method not found!