[Feature] Avoid duplicating assets and save server resources and storage space

The feature Media assets should be stored on a per-instance basis, rather than a per-user basis. This should be true for both uploaded media, as well as generated media. Let's say I take a 20-minute video with my phone. Then I share it with my family members, who save it to their phones, and who all have the Immich app installed. Now Immich will upload multiple copies of the same video to the server, and then re-encode this video multiple times, once for each user. Let's also say I have an External Library directory with 5000 photos of a family trip. I want to add that directory to each Immich account, so everyone can see those photos. Immich will now generate and store 5000 thumbnails for each user. Basically, there is a lot of resource waste, both in storage, and in generating encoded video, thumbnails, etc. I think a lot can be saved by checking if an asset (uploaded or generated) already exists somewhere on the server before uploading/generating. I think thi…