tkn hub install: Support --if-not-exists

[adambkaplan]

Feature request

For tkn hub install: add an option so that the install command does not fail if the catalog resource already exists in the cluster namespace.

Example:

$ tkn hub install task git-clone --version 0.9 --if-not-exists -n my-playground
Task git-clone(0.9.0) already installed in my-playground namespace

Use case

1. Pipelines in external systems (ex: Jenkins) that configure or deploy a namespace.

UI Example

$ tkn hub install task git-clone --version 0.9 --if-not-exists -n my-playground

[vinamra28]

this feature request should go in https://github.com/tektoncd/hub, hence moving it there