refactor(portal): the display of long_text for Interactive should be refactored to varchar(max_length)

[longbinlai]

For now, portal (for both data importing and model description),

There are three types of data that we aim to support for DT_STRING :

• CHAR (fixed_length): which is a fixed-length char array
• VARCHAR (length): which is a char array where each item can have variant length but cannot exceed the given length
• LONG_TEXT: which is an array of strings of arbitrary length

Now Interactive only supports VARCHAR(length), but portal uses DT_STRING, which has the semantic of LONG_TEXT, we must explictly tell user that it is VARCHAR, and allow them to specify the length upperbound, knowing that string exceeding that length will be cut off.

[longbinlai]

Also check this PR: alibaba/GraphScope#4367, for tracking the progress of type alignments on the backend engines.