Support `max_use` and `max_lifetime` backend parameters

[ulyssa]

Change summary

This draft PR adds new --max-use and --max-lifetime parameters to fastly backend create and fastly backend update, which can be used to specify the max_use and max_lifetime backend fields added in fastly/go-fastly#764.

All Submissions:

• [x] Have you followed the guidelines in our Contributing document?
• [x] Have you checked to ensure there aren't other open Pull Requests for the same update/change?

New Feature Submissions:

• [ ] Does your submission pass tests?
  • Not yet, the PR will need to have its go.mod/go.sum updated once there's a go-fastly release that includes fastly/go-fastly#764.

Changes to Core Features:

• [x] Have you written new tests for your core changes, as applicable?
  • Yes
• [ ] Have you successfully run tests with your changes locally?
  • Not yet, need to update go.mod/go.sum so that it builds.

User Impact

This will allow users to further control how connection pooling works in both Delivery and Compute once the API is updated to account for these upcoming fields.

Are there any considerations that need to be addressed for release?

None that I'm aware of.