SlackJsonTokenExtractor bug when access_token is not found in response body

[jaxony]

Hi there! I'm in the process of migrating Slack V1 App to V2 but ran into an error.

The code here expects that access_token is required, but in fact with Slack V2, access_token (meaning the "bot token") can be missing from the response object if the client only requested for "user-scoped permissions". See Slack oauth.v2.access docs.

scribejava/scribejava-core/src/main/java/com/github/scribejava/core/extractors/OAuth2AccessTokenJsonExtractor.java

Line 90 in 1247459

return createToken(extractRequiredParameter(response, OAuthConstants.ACCESS_TOKEN, rawResponse).asText(),

A proposed solution is to override the OAuth2AccessTokenJsonExtractor#createToken(String rawResponse) method in SlackJsonTokenExtractor, making the access token an optional parameter instead of a required one.

Happy to do the PR for this if the owner is happy with the proposed solution 😄