[FR]. Ability to get BLOB_ID for blob that has been created / modified by current transaction

[pavel-zotov]

This feature is needed to implement test for #8323
A transaction that has inserted or updated data into the blob column must have ability to obtain BlobID that was assigned to it (i.e. when blob has been materialized).
This feature will allow to make full-fledged test for above mentioned ticket, but also it might be useful in the future for some other purposes.