Support max_use and max_lifetime backend parameters

[ulyssa]

Change summary

This is a draft PR for adding support for two different backend fields that are going to be added to the API:

• max_use, which specifies how many times an HTTP keepalive connection can be used before it's no longer pooled.
• max_lifetime, which specifies in milliseconds an upper bound for how long an HTTP keepalive connection can be open before it's no longer used.

All Submissions:

• Have you followed the guidelines in our Contributing document?
• 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?
  • Yes

Changes to Core Features:

• Have you written new tests for your core changes, as applicable?
  • I didn't add any tests, since this just updates what can be passed in the struct to the API.
• Have you successfully run tests with your changes locally?
  • Yes

User Impact

This will allow using a newly exposed feature in both Delivery and Compute, which have added support for honoring these backend parameters in their connection pooling logic.

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