Add a storage layer for attachments

[lunny]

• This PR introduced a storage abstract layer to store attachments in local disk or minio(via s3 protocol). It's inspired by NOT MERGED: Add bucket storage support #9567 but with a smaller interface to handle the object storages. According minio's document, it should support to access any Amazon S3 compatible object storage.(https://docs.min.io/docs/golang-client-quickstart-guide.html)
• Changed mysql intergration tests to store attachments to a minio docker.
• It also introduced a sub command to migrate attachments to a new sotrage placement.

Below are the steps to migrate your attachments to a minio service.

• First, migrate local attachments to a minio server

./gitea migrate-storage --type=attachments --store=minio --minio-endpoint=localhost:9000 --minio-access-key-id=123456 --minio-secret-access-key=987654321 --minio-bucket=gitea --minio-location=us-east-1 --minio-base-path=attachments/ --minio-use-ssl=false

• Second, change your app.ini section attachment to change STORE_TYPE as minio and other information like above.

• Last, restart gitea. It's done.

Next steps (On other PRs):

• LFS support minio storage LFS support to be stored on minio #12518
• Avatars support minio storage Avatars and Repo avatars support storing in minio #12516
• Repo-Avatars support minio storage Avatars and Repo avatars support storing in minio #12516

[techknowlogick]

a few minor nits

[lunny]

It's ready to review now.

[lunny]

It's ready to review again now.

[techknowlogick]

Writing out my thoughts in case others thinking the same as me. I was thinking about suggesting renaming minio flags, and storage provider to be s3, as s3 is mostly generic, however having it specifically be minio is better because not all s3 providers act the same. (ex. DO Spaces only supports a legacy way of listing objects in a bucket). So by having it be minio means we can later add providers as needed in case there are any edge cases they have.

[lunny]

@techknowlogick Yeah. For a self-hosted private deployment, a self-hosted minio is a better chosen. For those public gitea instances, they may want s3 compitable services.

I kept a simple interface for ObjectStorage and not use an exist package so that future implementations for s3, DO or other public services are easy enough.

[mrsdizzie]

need to fix check