Introduce a AttachmentStore SPI

[chetanmeh]

Currently ArtifactStore implementation based on CouchDB stores the binaries as attachments to main document. For larger binaries (say > 1MB) it would be better to store them in some binary store. Further if we are deploying a custom OW setup in cloud then it would be better to store the binaries in cloud specific binary storage like S3/Azure BlobStore/IBM Object Storage

To support such a case we should introduce a new SPI in OW AttachmentStore. Some key points wrt to such an spi

1. Used internally by ArtifactStore and need not be exposed to layers above.
2. A default memory based implementation along with TCK to validate the contract
3. Later implement custom implementation for S3/Azure BlobStore/IBM Object Storage which can use the newly refactored PoolingRestClient and have the TCK test run against them
4. Allow using such a stored for CouchDbRestStore also via config

As an example MongoDBStore implementation uses GridFS to store attachements via GridFSAttachmentStore.

Would work on a PR to discuss the SPI interface and other aspects

[chetanmeh]

All planned work for this issue done. Closing the issue