[CDTOOL-1253] Move backend command under service

[rcaril]

Change summary

This PR moves the backend command under the service command. A alias for fastly backend is added here to prevent breaking changes.

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      (cached)
make test TEST_ARGS="-run TestBackendCreate ./pkg/commands/service/backend"
ok      github.com/fastly/cli/pkg/commands/service/backend      (cached)
make test TEST_ARGS="-run TestBackendList ./pkg/commands/service/backend"
ok      github.com/fastly/cli/pkg/commands/service/backend      1.086s
make test TEST_ARGS="-run TestBackendDescribe ./pkg/commands/service/backend"
ok      github.com/fastly/cli/pkg/commands/service/backend      1.052s
make test TEST_ARGS="-run TestBackendUpdate ./pkg/commands/service/backend"
ok      github.com/fastly/cli/pkg/commands/service/backend      1.096s
make test TEST_ARGS="-run TestBackendDelete ./pkg/commands/service/backend"
ok      github.com/fastly/cli/pkg/commands/service/backend      1.055s

Changes to Core Features:

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

User Impact

None, alias's have been created to prevent breaking changes.