What this PR does / why we need it:
Add pull, push scope hints before calling Copy for oras push and oras attach.
This can potentially save 2 requests for each command.

oras push

oras push $myreg/$myrepo:$mytag --debug

Before

request #0: HEAD manifest digest - 401
request #1: POST token (scope: pull) - 200
request #2: HEAD manifest digest - 404
request #3: HEAD config ({}) - 404
request #4: POST config ({}) - 401
request #5: POST token (scope: pull, push) - 200
request #6: POST config ({}) - 202
request #7: PUT config ({}) - 201
request #8: PUT manifest tag - 201

After

request #0: HEAD manifest digest - 401
request #1: POST token (scope: pull) - 200
request #2: HEAD manifest digest - 404
request #3: HEAD config ({}) - 404
request #4: POST config ({}) - 202
request #5: PUT config ({}) - 201
request #6: PUT manifest tag - 201

oras attach

oras attach myreg/myrepo:mytag --artifact-type application/test foo --debug

Before

request #0: HEAD subject tag - 401
request #1: POST token (scope: pull) - 200
request #2: HEAD subject tag - 200
request #3: HEAD manifest digest - 404
request #4: HEAD config ({}) - 200
request #5: HEAD layer - 404
request #6: POST layer - 401
request #7: POST token (scope: pull, push) - 200
request #8: POST layer - 202
request #9: PUT layer - 201
request #10: PUT manifest tag - 201

After

request #0: HEAD subject tag - 401
request #1: POST token (scope: pull) - 200
request #2: HEAD subject tag - 200
request #3: HEAD manifest digest - 404
request #4: HEAD config ({}) - 200
request #5: HEAD layer - 404
request #6: POST layer - 202
request #7: PUT layer - 201
request #8: PUT manifest tag - 201

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #1084

Please check the following list:

• Does the affected code have corresponding tests, e.g. unit test, E2E test?
• Does this change require a documentation update?
• Does this introduce breaking changes that would require an announcement or bumping the major version?
• Do all new files have an appropriate license header?