[CDTOOL-1317] Add Support for the max_use and max_lifetime backend parameters

[rcaril]

Change summary

This PR adds new --max-use and --max-lifetime parameters to fastly service backend create and fastly service backend update, which can be used to specify the max_use and max_lifetime backend fields.

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?

make test TEST_ARGS="-run TestBackendCreate ./pkg/commands/service/backend"
ok      github.com/fastly/cli/pkg/commands/service/backend      1.204s

make test TEST_ARGS="-run TestBackendList ./pkg/commands/service/backend"
ok      github.com/fastly/cli/pkg/commands/service/backend      1.130s

make test TEST_ARGS="-run TestBackendDescribe ./pkg/commands/service/backend"
ok      github.com/fastly/cli/pkg/commands/service/backend      1.071s

make test TEST_ARGS="-run TestBackendUpdate ./pkg/commands/service/backend"
ok      github.com/fastly/cli/pkg/commands/service/backend      1.133s

make test TEST_ARGS="-run TestBackendDelete ./pkg/commands/service/backend"
ok      github.com/fastly/cli/pkg/commands/service/backend      1.065s

Changes to Core Features:

• Have you written new tests for your core changes, as applicable?
• Have you successfully run tests with your changes locally?

[philippschulte]

The implementation looks good to me overall. One small consistency nitt: CreateCommand.MaxUse and CreateCommand.MaxLifetime should probably be unexported as maxUse and maxLifetime, matching nearby fields like maxConn, firstByteTimeout, and healthCheck. They do not appear to need to be exported. The update command already uses exported fields consistently, so no issue there.

[philippschulte]

Thank you!