Implement gwctl get gatewayclass

[jongwooo]

What type of PR is this?

/kind feature
/area gwctl
/cc @gauravkghildiyal

What this PR does / why we need it:

Implement gwctl get gatewayclass.

Output Columns	Description	Visibility
NAME	Name of the GatewayClass
CONTROLLER	Controller managing the GatewayClass
ACCEPTED	Whether the GatewayClass is accepted by the controller
AGE	Age of the Policy CRD
GATEWAYS	Count of Gateways using this GatewayClass	-o wide
DESCRIPTION	Description from the GatewayClass	-o wide

The -o wide format is not yet implemented, so the columns GATEWAYS, DESCRIPTION are excluded.

Which issue(s) this PR fixes:

Fixes #2846

Does this PR introduce a user-facing change?:

Implement `gwctl get gatewayclass`

[gauravkghildiyal]

/assign

[gauravkghildiyal]

Neat! Thanks

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: gauravkghildiyal, jongwooo

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• gwctl/OWNERS [gauravkghildiyal]

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