feat(svix-server): server healthchecks

[y-nk]

Motivation

As discussed in the community slack, the server lacks of healthcheck which is easily available for instrumentalisation (docker-compose, kubernetes, etc)

Solution

I'm personally inclined not to have a command in the svix-server binary, mostly because it would not self-introspect a running server but rather "just fetch the healthcheck endpoint", therefore adding curl to the docker image seems lighter and less invasive imho.

I've implemented both solutions:

1. a healthcheck command which makes a HEAD /v1/health request and returns Ok() on 200 (error otherwise)
2. added curl in the server's Dockerfile
3. implemented both tests in the docker-compose

Note

1. As requested i've also added the healthcheck commands and condition status of the dependencies in the docker-compose file
2. Please be gentle in the review as I'm not fluent in rust