Move instance credentials to shared library

[timj-hh]

Summary

This change moves remaining credentials module after [this pull request] to the shared library. This is needed for ECS frontend, EC2 client, ECR factory. Usages in agent/ have been accordingly updated.

Implementation details

• Move and re-merge agent/credentials/instancecreds to ecs-agent/credentials/instancecreds and agent/credentials/providers to ecs-agent/credentials/providers
• Replace uses of seelog to logger
• go mod init && go mod vendor ran in ecs-agent and agent directories including aws-vpc-cni-plugins among other things.

amazon-ecs-agent/ecs-agent/credentials now looks like:

credentials
├── generate_mocks.go (Existed)
├── instancecreds
│   ├── instancecreds.go
│   ├── instancecreds_linux.go
│   ├── instancecreds_test.go
│   ├── instancecreds_unsupported.go
│   └── instancecreds_windows.go
├── interface.go (Existed)
├── manager.go (Existed)
├── manager_test.go (Existed)
├── mocks (Existed)
│   └── credentials_mocks.go (Existed)
└── providers
    ├── credentials_filename_linux.go
    ├── credentials_filename_unsupported.go
    ├── credentials_filename_windows.go
    ├── rotating_shared_credentials_provider.go
    └── rotating_shared_credentials_provider_test.go

Testing

make test and make run-integ-tests

New tests cover the changes: no (existing tests pass)

Description for the changelog

Move instance credentials and credential provider to shared library

Does this PR include breaking model changes? If so, Have you added transformation functions?
No

Licensing

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.