intra-step leasing: Add lease proxy server specs in prowgen

[danilo-gemoli]

This is the preparatory work that ci-operator requires in order to run a small lease proxy server.

We are working on the intra-step leasing capability that will allow a step from the ci-operator step registry to acquire lease from boskos. As of today, ci-operator acquire any leases required by a workflow and then run the test. With the new feature we are about to introduce, a step could also do that by sending requests to ci-operator that will, in turn, forward them to boskos.

A typical scenario looks like so:

1. ci-operator is supposed to run the e2e-aws-ovn test:

- as: e2e-aws-ovn
  skip_if_only_changed: (^(docs|upi|hack)/)|((^|/)OWNERS(_ALIASES)?$)|((^|/)[A-Z]+\.md$)|(^images/(baremetal|libvirt|openstack))|(^cluster-api/providers/([^a]|a[^w]|aw[^s]).*/)|(/agent/)
  steps:
    cluster_profile: aws-4
    env:
      AWS_INSTALL_USE_MINIMAL_PERMISSIONS: "yes"
      USER_TAGS: |
        keyA valueA
        key2 value2
        keySpecial v@lu3-Special_.:/=+
        keyD valueD spaces
    workflow: openshift-e2e-aws

2. The test uses a cluster_profile, therefore ci-operator runs a small HTTP server that acts as a lease proxy server, actually forwarding any requests to boskos.
3. The workflow openshift-e2e-aws executes.
4. Every step of the workflow has the CI_OPERATOR_POD_IP environment variable set, that it might uses to send requests to the proxy run by ci-operator.
5. The ipi-install-install step runs and tries to acquire a lease from the proxy.
6. ci-operator receives the request, forwards it to boskos and finally returns the result back to the ipi-install-install step.

ipi-install-install.sh step
|
HTTP POST `/acquire?type=aws-1-qe-quota-slice&count=1`
|
+----> lease-proxy-server run by `ci-operator`
          |
          HTTP POST `/acquire?type=aws-1-qe-quota-slice&count=1`
          |
          +----> `boskos`