feat: add iam, aws config file, minio config file configurable credentials for s3 blob storage

docs/reference/blob/s3.md

@@ -13,6 +13,7 @@ storage.blob.s3 {
     # optional
     region eu-central-1
     object_prefix maddy/
+    creds access_key
 }
 ```
 
@@ -26,6 +27,7 @@ storage.imapsql local_mailboxes {
         secret_key "..."
         bucket maddy-messages
         region us-west-2
+        creds access_key
     }
 }
 ```
@@ -69,3 +71,16 @@ in some manuals.
 String to add to all keys stored by maddy.
 
 Can be useful when S3 is used as a file system.
+
+**Syntax:** creds _string_ <br>
+**Default:** access_key
+
+Credentials to use for accessing the S3 Bucket.
+
+Credential Types:
+ - access_key: use AWS access key and secret access key 
+ - file_minio: use credentials for Minio present at ~/.mc/config.json
+ - file_aws: use credentials for AWS S3 present at ~/.aws/credentials
+ - iam: use AWS IAM instance profile for credentials.
+
+By default, access_key is used with the access key and secret access key present in the config.