`workload get -o` and `workload list -o` accepts `yaml` and/or `yml`

[heyjcollins]

Description of problem

Workload get and workload list commands acceptyaml as an --output value today.
Because yml is a valid extension for the same file-type, we should accept yml and/or yaml

Proposed solution

Given I have a workload named foo
When I run `tanzu apps workload get foo -o yml`
Then the output from the command is exactly the same as if I ran `tanzu apps workload get foo -o yaml`

Given I have created workloads on my target cluster
When I run `tanzu apps workload list -o yml`
Then the output from the command is exactly the same as if I ran `tanzu apps workload list -o yaml`