Consider a PolicyConfig struct instead of JSON string to set bucket policy

[HJLebbink]

Propose refactor of JSON string to configure bucket policy into a PolicyConfig struct.

See

minio-rs/src/s3/builders/set_bucket_policy.rs

Line 35 in 112c0ae

pub(crate) config: String, //TODO consider PolicyConfig struct

Once such a struct is in place, tests can be updated to use struct equality tests.
See

minio-rs/tests/test_bucket_policy.rs

Line 49 in 48da763

// TODO create a proper comparison of the retrieved config and the provided config