Add getBucketPolicy, putBucketPolicy, deleteBucketPolicy & getBucketPolicyStatus

[c4spar]

I will open a PR for this too 😉

[c4spar]

I'm wondering if we should move the getBucketPolicy, putBucketPolicy, deleteBucketPolicy && getBucketPolicyStatus methods to the S3Bucket class and rename them to getPolicy, putPolicy, deletePolicy & getPolicyStatus? Currently i have added them to the S3 class becasue the original aws sdk has these methods also on the S3 class but i think the S3Bucket class would be also a good place.

I would also like to add a getBucket method, than we can easily retrieve/switch bucket instances and call buckets with:

const s3 = new S3({
  accessKeyID: Deno.env.get("AWS_ACCESS_KEY_ID")!,
  secretKey: Deno.env.get("AWS_SECRET_ACCESS_KEY")!,
  region: "us-east-1",
  endpointURL: Deno.env.get("S3_ENDPOINT_URL"),
});

s3.getBucket("my-bucket").putPolicy({...})