S3: add more S3 matchers.

[bboure]

s3Object() supports toExist(), toExistAndMtchSnapshot(), toExistAndMatchInlineSnapshot() and toExistAndMatchObject().

However, toExistAndMatchObject() only works with JSON content.
Snapshots work with any kind of content, but we should add a way to match on string literals:

expect(s3Object(...)).toExistAndEqual('hello world!');

Other useful possible matchers:

• toExistAndHaveContentType('image/jpeg');
• toExistAndHaveAclMatching<AclType>({...});
• etc.