Rename or Reconsider ELASTIC_APM_EXTENSION_TIMEOUT_SECONDS env Variable

[astorm]

The ELASTIC_APM_EXTENSION_TIMEOUT_SECONDS env variable allows users to set timeout values

apm-aws-lambda/apm-lambda-extension/extension/http_server.go

Lines 55 to 60 in 63afa0a

	timeout := time.Duration(config.dataReceiverTimeoutSeconds) * time.Second
	s := &http.Server{
	Addr: config.dataReceiverServerPort,
	Handler: &handler,
	ReadTimeout: timeout,
	WriteTimeout: timeout,

We should either

1. Rename this variable to more clear express it's intent, @felixbarny suggested ELASTIC_APM_EXTENSION_TIMEOUT_SECONDS

2. Reconsider whether it's needed

This variable was created early in development, and the default value picked was "something a little bit longer than the max time an agent may keep a connection open. Depending on how consistent the agents are in the env naming, we may be able to look at the value set for ELASTIC_APM_API_REQUEST_TIME (or use the default of 10s when it's not set) and just add an additional second to ensure the HTTP Server's timeout is a little bit longer than the max agent requests time.