feat(gatsby): add gatsby plugin configure command

[mxstbr]

NOTE: This is based on #27703 so until that is merged you'll also see those changes here.

TODO

• Write to gatsby-config.js using the GatsbyPlugin provider from gatsby-recipes
• Figure out how to handle all the possible Joi types (enums, arrays, objects, etc.) with the enquirer inputs
• Hide token/secret values in inputs (feat(gatsby): more gatsby plugin configure additions #27736)
• Fix the initial (default) value only supporting strings (feat(gatsby): more gatsby plugin configure additions #27736)
• Validate options
• Only write non-default values to gatsby-config.js
• Add tests (probably copy the ones Matt wrote for create-gatsby)
• Hook it up as an actual command behind the feature flag for testing (once feat: Add "gatsby plugin" command #27725 lands)

How to work on this

# Terminal tab 1
$ yarn workspace gatsby watch

# Terminal tab 2
$ yarn workspace gatsby-cli watch

# Terminal tab 3
# Note: this needs to be in a gatsby site that is either in gatsby/packages (I have it in gatsby/packages/test-site) or one that runs gatsby-dev-cli
$ GATSBY_EXPERIMENTAL_PLUGIN_OPTION_VALIDATION=true GATSBY_EXPERIMENTAL_PLUGIN_COMMANDS=true node ../gatsby-cli/lib/ plugin configure gatsby-source-contentful

[ch17926]

[ascorbic]

Are you able to not include create-gatsby in this?