Fix OAuth2 security schemes for OpenAPI importer

[gazoakley]

Closes #3093
Closes #4095

The existing importer for OpenAPI requires specifying scheme: bearer for OAuth2 security schemes, but unfortunately this violates the OpenAPI specification (and Insomnia will show warnings when using the schema editor). In this PR:

• Fixes importing OpenAPI specs for OAuth2 security schemes
• Updates the test fixtures to comply with the OpenAPI specification for OAuth2
• Updates the test result fixtures for Petstore tests (these previously didn't import correctly due to the above issue)
• Sets the Redirect URL for OAuth2 requests to a variable - this is a required parameter for Amazon Cognito/Okta and most/all IdPs, so being able to set it easily for all requests (as is possible for Client ID/secret) simplifies using Insomnia

changelog(Fixes): For OpenAPI documents, OAuth2 security scheme components no longer require the scheme to be set to bearer

[gazoakley]

Doh - lint issue! Updated and rebased

[martin-thoma]

Is there anything missing here? Or is it only the review that still needs to be done?

[dimitropoulos]

@MT-Cash we're going to look at this on the stream today in a little bit. see the #stream channel on the insomnia community slack if you'd like to be there or watch (no pressure whatsoever).

[gatzjames]

👏🚀 Great UX improvement and spec compliance! Did some extra testing and seems to work as expected! We should simplify the scheme logic when we revisit this but works fine for now!