port `graphiql_toolbar_buttons` from core plugin

[josephfusco]

User Story

As a WordPress developer, I would like to be able to actions/filter various parts of the GraphiQL IDE to provide customized functionality.

Acceptance Criteria

• [x] new action/filter names is prefixed with wpgraphqlide_
• [x] existing functionality of action/filter is honored
• [x] ported action/filter is documented

Reference

• Core plugin
• Existing documentation

[josephfusco]

Closed by https://github.com/wp-graphql/wpgraphql-ide/pull/60

[josephfusco]

Closing as not planned - in favor of https://github.com/wp-graphql/wpgraphql-ide/issues/109