Implement gwctl get gateways

[jongwooo]

What type of PR is this?

/kind feature
/area gwctl
/cc @gauravkghildiyal

What this PR does / why we need it:

Implement gwctl get gateways.

Output Columns	Description	Visibility
NAME	Name of the Gateway
CLASS	Class of the Gateway
ADDRESSES	Addresses of the Gateway (displayed using +n more)
PORTS	Ports exposed by the Gateway
PROGRAMMED	Whether the Gateway is programmed
AGE	Age of the Gateway
POLICIES	Count of policies affecting this Gateway	-o wide
HTTPROUTES	Count of HTTPRoutes that are attached to this Gateway	-o wide

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

Which issue(s) this PR fixes:

Fixes #2778

Does this PR introduce a user-facing change?:

Implement `gwctl get gateways`

[gauravkghildiyal]

This looks great! Thanks @jongwooo!

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• gwctl/OWNERS [gauravkghildiyal]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment