feat: explore select

[LukaFontanilla]

This PR adds the support for multi explore to Explore Assistant. It pulls from this PR, except is adopted for the new EA UI/UX. Namely:

• Explore Select is accessed from the Settings Widget
• Default Explore is from Redux state, if that is not defined it pulls from environment variables
• Explore dropdown is populated via a BigQuery table
• Terraform for provisioning BigQuery sample table (for dynamic explore query samples)
• BigQuery loader update to allow admins to configure new samples without redeploying extension