add: ability to load configuration from stdin

[kameshsampath]

Honor stdin for passing configuration when doing k3d cluster create -c

What

With this PR we can create k3d clusters with configuration from CLI

e.g.

cat <<EOF | $EXE cluster create "test-stdin" --config=-
apiVersion: k3d.io/v1alpha4
kind: Simple
metadata:
  name: test-stdin
servers: 1
agents: 1
image: rancher/k3s:v1.24.3-k3s1
EOF

Why

#1123

Implications

It only updates the cmd package with extra logic to check if the config is from stdin or from file. It also added extra iostreams helper under cmd/util

@all-contributors please add @kameshsampath for ideas and code

[kameshsampath]

@iwilltry42 - I added a test script in the tests inline with others, LMK if that looks good or needs change. Happy to update

[iwilltry42]

Fixed some details so that tests and linting pass.

Thanks a lot for your contribution @kameshsampath :)