Connection string support

[Xuanwo]

Allow user to init storage backend like what we does for database:

• fs:///path/to/dir
• s3://s3.amazonaws.com/bucket/path/to/dir?aws_access_key_id=xxx&aws_secret_access_key=xxx&region=xxx

[Xuanwo]

ping @bohutang for more feature requests.

[bohutang]

Cool!

For databend, the s3://s3.amazonaws.com/bucket/path/to/dir URL will not be passed to OpenDAL directly.
And the CREDENTIALS will parse into a struct(Stage) in SQL layer, then call OpenDAL API like:

let operator = opendal::services::s3::Backend::build()
                    .bucket(bucket)  -- we know bucket
                    .credential(credential) -- we know the credential
                    .finish()
                    .await

All, the current higher priority for databend tasks are(These two are enough for now):
#37 and #12