Support external Git LFS object storage

While Bitbucket is able to work with LFS objects stored externally, it however, doesn't have the ability to recognize the availability of the files. The UI shows that the file is not present when we are in either the Compare page or the Pull Request page.

For example, if the LFS objects are stored in a Jfrog Artifactory, it is tracked in Git LFS and the repository has a .lfsconfig file pointing to the Artifactory repository. When viewing the compare page on the UI, Bitbucket is unable to find the file in the local LFS storage and throws a 404 error with the following line logged:

{"objects":[{"oid":"d638dd7b5cd219ad8bc138dbe54245a65593f32fa56330542f157a52e7728c49","size":70144,"error":{"message":"Object does not exist on the server","code":404}}]}

This can be misleading to end users as they will think the file has been deleted on the server, when in actual fact, it is stored outside of Bitbucket in Artifactory (using Git LFS) and cannot be compared.

Bitbucket should be able to recognize that the files are in an external object storage and link them when in the Compare page.