This PR introduces two new features
- AWS STS compatible STS API named AssumeRoleWithClientGrants
```
POST /?Action=AssumeRoleWithClientGrants&Token=<jwt>
```
This API endpoint returns temporary access credentials, access
tokens signature types supported by this API
- RSA keys
- ECDSA keys
Fetches the required public key from the JWKS endpoints, provides
them as rsa or ecdsa public keys.
- External policy engine support, in this case OPA policy engine
- Credentials are stored on disks
Previously we used allow bucket policies without
`Version` field to be set to any given value, but
this behavior is inconsistent with AWS S3.
PR #5790 addressed this by making bucket policies
stricter and cleaner, but this causes a breaking
change causing any existing policies perhaps without
`Version` field or the field to be empty to fail upon
server startup.
This PR brings a code to migrate under these scenarios
as a one time operation.
- remove old bucket policy handling
- add new policy handling
- add new policy handling unit tests
This patch brings support to bucket policy to have more control not
limiting to anonymous. Bucket owner controls to allow/deny any rest
API.
For example server side encryption can be controlled by allowing
PUT/GET objects with encryptions including bucket owner.
