Add label filtering (-l) for get commands

[deszhou]

What type of PR is this?

What this PR does / why we need it:

Which issue(s) this PR fixes:

Fixes #2890

Does this PR introduce a user-facing change?:

Add label filtering (-l) for get commands

• Correct label format output

➜  gwctl get httproutes -l app=backend
NAMESPACE  NAME     HOSTNAMES        PARENT REFS  AGE
default    backend  www.example.com  1            30d

• Error label format output

➜  gwctl get httproutes -l aps=dsd,adsa,=ds
Unable to find resources that match the label selector "aps=dsd,adsa,=ds": found '=', expected: identifier after ','