feat: global config cli

[lkingland]

**** Please note this PR is currently still a Work In Progress ****

Changes

• 🎁 Adds Global System Configuration via func config [get|set|unset|list] commands

Taking place of the commands promoted up one level in #1620 (git,volumes, labels etc) is a new config subcommand which is specifically for Global Config default settings. See #1620 for a discussion of the reasoning behind promoting the metadata commands to the root and repurposing the config subcommand to be dedicated to global defaults.

/kind enhancement

Fixes #901

Release Note

Adds a CLI for managing global confuration settings via the 'config' subcommand

Docs

[knative-prow]

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[knative-prow-robot]

PR needs rebase.

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