Configurable timeout for "wait-for-service" for services
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## Description
I'd like to use `services` for dependencies, that start rather slowly, but there is no way to do that reliably, if I can't configure the waiting timeout.
context:
```
Using Docker executor with image docker:18 ...
Starting service docker:18.09-dind ...
Authenticating with credentials from $DOCKER_AUTH_CONFIG
Pulling docker image docker:18.09-dind ...
Using docker image sha256:e4157102c815f9ed8fec6118171866b13a331b6329ff8f85017f0bf7652917e9 for docker:18.09-dind ...
Waiting for services to be up and running...
*** WARNING: Service runner-abc-project-51-concurrent-0-docker-0 probably didn't start properly.
Health check error:
service "runner-abc-project-51-concurrent-0-docker-0-wait-for-service" timeout
```
in reality, `dind` starts fast - this is just an example
## Proposal
This just an example, what it could look like
```yml
job-build:
services:
- name: docker:18.09-dind
alias: docker
startup_timeout: 60
```
the `wait-for-service` would then simply wait for the given amount of time, instead of the system default
issue