Add volume attachments info to the API and CLI

[r4victor]

Closes #2278

This PR:

• Adds Volume.attachments – the list of instances that the volume is attached to with attachment data.
• Deprecates Volume.attachment_data in favor of Volume.attachments[n].attachment_data.
• Replaces volumes<->instances many-to-many relationship with a many-to-one one-to-many relationship via an association object: basically extends volumes_attachments table with attachment_data column and replaces the implicit many-to-many via a secondary volumes_attachments with explicit relationship with volumes_attachments. This is necessary since attachment_data should belong to attachment and not to volume as before. A preliminary for multi-attach volumes with attachment data.
• Adds ATTACHED column to the dstack volume -v output with fleet names the volume is attached to. Currently instance nums are not showed to keep output concise but they can be added if requested.

✗ dstack volume -v
 NAME             BACKEND  REGION             STATUS  ATTACHED  CREATED     ERROR 
 my-volume        aws      eu-west-1          active  my-dev    5 days ago        
                           (eu-west-1a)                                           
 my-gcp-volume-2  gcp      europe-west4       active  -         5 days ago        
                           (europe-west4-c)