[core] support adding blob column through comments

[steFaiz]

Purpose

This PR is about to replace #7921

This PR introduces a comment format to mark an adding column as BlobType. So that these engines which do not have BlobType can also add blob columns.
The rule is:

1. define 2 directives: __BLOB_FIELD, __BLOB_DESCRIPTOR_FIELD
2. user comments are seperated by ;: e.g. __BLOB_FIELD; this is a blob field, The starting directives will be trimmed.
3. blob-field and blob-descriptor-field are immutable, and will be atomically modified during Alter Table Add Column and Drop Column.

NOTE:

1. we do not permit updating any existing non-blob fields to blob type nor updating any blob field to non-blob type.
2. we do not permit adding an blobType field without directives, since we don't know how to store them.

Tests

ITCases and UnitTests

[JingsongLi]

+1