Support parameterized CMPs

[crenshaw-dev]

Summary

Implement parameterized CMPs (see the proposal: https://github.com/argoproj/argo-cd/blob/master/docs/proposals/parameterized-config-management-plugins.md)

Proposal

I think each of these should be its own PR.
1 -> 2 must be done in series.
3 can be done in parallel with 1 -> 2.
4 can be done in parallel with 1 -> 2 and 3, but parameterization must wait until 1-3 are finished.

• 1) add a parameters announcement endpoint to the Server with static param handling
  • add a GetParameters service to cmpserver/plugin/plugin.proto - it should be a streaming request like the existing ones
  • run make protogen
  • add the GetParameters handler to cmpserver/plugin/plugin.go - it should simply return the items in the static param announcement config (https://github.com/argoproj/argo-cd/blob/master/docs/proposals/parameterized-config-management-plugins.md#how-will-the-configmanagementplugin-spec-change)
  • add tests to make sure the endpoint serves up the static parameters correctly
• 2) add dynamic param handling to parameters announcement endpoint
  • call configured announcement command and append parameters announcements to any static announcements
  • if 3 has been completed, make sure parameters from the Application spec are sent as env vars to the announcement endpoint
  • add tests to make sure 1) parameters are pulled from the commands, 2) parameters are appended to static params, 3) (if applicable) parameters from the Application spec are available to the command as env vars, and 4) errors from the command are handled
• 3) send parameters from Application spec to the CMP server endpoints
  • define the parameter spec as proto in cmpserver/plugin/plugin.proto (it's described as Go here: https://github.com/argoproj/argo-cd/blob/master/docs/proposals/parameterized-config-management-plugins.md#how-will-the-configmanagementplugin-spec-change)
  • add the parameter as a repeated field in ManifestRequestMetadata
  • run make protogen
  • send the params as a JSON env var and convenience env vars
    to the commands run by the CMP (if 2 has been finished, make sure params are sent to the announcements endpoint)
  • add tests to verify that the params arrive as expected in the env vars
• 4) make a Helm CMP based on native Helm config management tool
  • try to do so with a command rather than a custom CMP server