Tekton pipelines should support using a network proxy

If the cluster is running in an environment where a proxy is required to access the external internet the Tekton tasks should support using the proxy config.

**Brute force approach:**

Use the pattern we already have in place:

- Provide the proxy details in a secret (use a secret because some proxies require authentication)
- Add the proxy secret as an optional dependency of the task(s)
- If the proxy configuration is provided, each task will need to use the values as appropriate

Design decision:

Which tasks should reference the proxy secret?
- [ ] only the Setup task (similar to a pattern we already employ)

+ secret can be referenced in one place
- requires a number (http and https proxy url and potentially different credentials for each) of parameters to be passed to every task

- [ ] each task references the secret and behaves accordingly

Alternatives:
- A cluster-wide proxy can be configured but this appears to only be for system components - https://docs.openshift.com/container-platform/4.6/networking/enable-cluster-wide-proxy.html
- Use Istio with Tekton and use egress proxy - https://istio.io/latest/docs/tasks/traffic-management/egress/http-proxy/

Related story:
- [Provide a guide for configuring workloads to use an external proxy](https://github.com/cloud-native-toolkit/planning/issues/837)