Add support for opening a BLOB for incremental I/O

[noteflakes]

The idea is to have a file like API (https://www.sqlite.org/c3ref/blob_open.html):

blob = db.open_blob(db_name, tbl_name, column_name, rowid)
blob.size #=> returns total blob size
blob.offset #=> 0
blob.read(12) #=> "blahblahblah"

• We maintain an offset and increment it as we read or write.
• Add the possibility to reopen using sqlite3_blob_reopen.