Move gmail object from children to parent GmailBaseTool

[uthmanmoh]

Before we were repeating the following code in each subclass:

const auth = await this.getAuth();
const gmail = google.gmail({ version: "v1", auth });

This was pointless because we had all the params already in the superclass, so this PR moves the gmail object to the parent class so children can just access it through this.gmail