Create new blobs id column with auto-sequencing

We need to create a new bigint ID column on the the blobs table to enable standard ID BBM operations and to be able to tackle the plan for Accept and record unknown media types (&13805) . The blobs table lacks a traditional sequential ID column, making it incompatible with standard background migration patterns.

As mentioned in : #1248 (comment 1876873852)

We need to manually manage the id field and the sequence, and another caveat is that postgres can't logically enforce uniqueness on a column across partitions unless the column in question is part of the partition key, which id is not. Thankfully, nextval('my_sequence') was designed to always return an incremented value regardless of being inside a transaction or not, so our procedure to add unique IDs should not generate any duplicates.