Authenticate Slack Integration

After the user installs/authorizes the Spot Connect Slack app in their workspace, they will be redirected back to the Slack integration page in the Fylamynt web app with an authorization code. The web app will then call this API endpoint with that 'code' in the query parameter. Instead, if the user rejects to authorize the Fylamynt Slack app in their workspace, the API request from the web app will contain the 'error' code (e.g., error=access_denied) returned from Slack. The following code processes the request and returns success or failure response to the web app. Slack instance alias is part of 'state' query parameter, previously 'state' query parameter contained subdomain.