feat(graphiql): add configurable credentials behavior

[benjie]

(Maintainer note: accidentally closed #1386 by merging another PR; then couldn't rebase #1387, so opened this one. Author is @eddiewang)

Description

This PR adds the ability to set the Request credential behavior on the GraphiQL instance built into Postgraphile. This allows users that rely on cookies to have a seamless experience querying in GraphiQL.

Performance impact

• Negligible, adds an extra option in the config.

Security impact

• Should only be enabled in dev-mode / unknown

• My code matches the project's code style and yarn lint:fix passes.
• I've added tests for the new feature, and yarn test passes.
• I have detailed the new feature in the relevant documentation.
• I have added this feature to 'Pending' in the RELEASE_NOTES.md file (if one exists).
• If this is a breaking change I've explained why.