feature request: set sidecar_task globally

### Nomad version
0.12.5

### Operating system and Environment details
linux

### Problem
For our envoy sidecar we are using a custom docker image as well as setting up some environment variables, results in a big chunk needs to be added everytime we declare a consul connected service. This 1. kinda pollutes the job file and 2. make management of the sidecar_task a bit challenging.

So would like to have something like `sidecar-task-defaults`, that would declare the default sidecar-task in one go.

No files or tests are named. Start by reviewing Nomad's service and sidecar_task job configuration and how Consul-connected services are declared; define the scope and precedence of a global sidecar-task default, with completion shown by avoiding repeated custom Docker image and environment-variable blocks.

Written by the indexing model from the issue text.