Checking for object existence

[Palladinium]

Is your feature request related to a problem? Please describe.
There is currently no way to check whether an object exists in a bucket.
The README has a link to an exists function, but the method doesn't actually exist:

rust-s3/README.md

Line 77 in 5105a1b

| `exists` | [async](https://docs.rs/rust-s3/latest/s3/bucket/struct.Bucket.html#method.exists)|

Describe the solution you'd like
Adding an exists method to the Bucket API.

Describe alternatives you've considered
I've tried using get_object or head_object and checking for the error variant, but I have no idea which S3Error variant is returned when the object doesn't exist vs when anything else goes wrong with fetching the object's data.

[Palladinium]

It looks like the error value returned when an object doesn't exist is S3Error::Http(404, _).
It'd still be nice to get this as its own feature.
Note that without the ListBucket permission, you will always get a 403 instead, and you tell whether an object doesn't exist or you just can't access it - this is by design in S3's API.