add ability to install Tekton Pipelines, Triggers, Chains, Dashboard

Feature request

Why don't we add support for deploying Tekton Pipelines, Chains, Dashboard via tkn CLI? IMHO, it'd be great if I could type:

$ tkn install cd
$ tkn install triggers
$ tkn install chains
$ tkn install dashboard
...

Use case

We use plain YAML manifests to deploy these applications. It'd be great to add this installation logic into the tkn CLI so that the tkn CLI would become an only needed application from the user's perspective. IMHO, who has tkn CLI, should do anything without requiring the knowledge of installations these tools.

UI Example

Like istioctl did, we might do the same for tkn CLI. There are two things that we have to consider.

• How do users set override values for the installation of each application?

Maybe we can set additional flags like --dry-run to output all the manifest that we're going to apply, like kubectl approach. Or, we can accept some override values like values.yaml for Helm, then use it for overriding the default ones.

• Which method should we use under the hood?

We can use executables of kubectl or Helm directly, or SDKs for each of them.

Start at the tkn CLI entry point and review how a new install command would expose Tekton Pipelines, Triggers, Chains, and Dashboard deployment. Resolve the requested override and execution approach, then define completion as working install subcommands that deploy the selected application manifests.