Enable PKCE support for Salesforce integration

PKCE is a security extension for the OAuth 2.0 protocol in Salesforce that many organizations are using. Intercom’s Salesforce app doesn't support PKCE currently. If an organization has PKCE enabled for their Salesforce instance, that means they aren’t able to install and use the native Salesforce app in Intercom. This is a global setting that can’t be toggled on and off for each app. Intercom needs to update the integration to support clients that require this extra layer of security.