Make schema refresh timeout configurable via env var

[zachliu]

What type of PR is this?

• [ ] Refactor
• [x] Feature
• [ ] Bug Fix
• [ ] New Query Runner (Data Source)
• [ ] New Alert Destination
• [ ] Other

Description

In certain situations, especially when using managed (or "serverless") data services such as AWS Athena, it may take more than 300 seconds to refresh a single data source (pinging the API for all schemas from the cloud platform). We need longer timeout in these situations.

How is this tested?

• [ ] Unit tests (pytest, jest)
• [ ] E2E Tests (Cypress)
• [x] Manually
• [ ] N/A

Related Tickets & Documents

It solves https://github.com/getredash/redash/issues/5402

Mobile & Desktop Screenshots/Recordings (if there are UI changes)