Support HTTP Proxy Settings in InstallSteward

[anothertobi]

Context

When applying the manifest returned by an installURL to a cluster which requires an HTTP proxy to communicate with the outside world, the deployment will fail to get ready.
It would be nice to have an additional query parameter which could be used to pass HTTP proxy settings to the lieutenant API which would then be rendered into the manifest returned by the installURL.
As a workaround, I patched the returned manifest locally before applying it to the cluster.

Affected code: https://github.com/projectsyn/lieutenant-api/blob/master/pkg/service/steward.go

Alternatives

An alternative implementation could be the usage of cluster facts for HTTP proxy settings.