[CDTOOL-1258] Move imageoptimizerdefaults commands under service

[rcaril]

Change summary

This PR moves the imageoptimizerdefaults command under the service command. A alias for fastly imageoptimizerdefaults 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 TestImageOptimizerDefaultsUpdate ./pkg/commands/service/imageoptimizerdefaults"
ok      github.com/fastly/cli/pkg/commands/service/imageoptimizerdefaults       1.117s
 make test TEST_ARGS="-run TestImageOptimizerDefaultsGet ./pkg/commands/service/imageoptimizerdefaults"
ok      github.com/fastly/cli/pkg/commands/service/imageoptimizerdefaults       1.086s

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, an alias has been made.

[anthony-gomez-fastly]

LGTM