Add Expression Index creation (RM #4260)

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Issue migrated from Redmine: https://redmine.postgresql.org/issues/4260
Originally created by Michel Feinstein at 2019-05-15 06:00:59 UTC.

Currently (version 4.6) there's no way to create an Expression Index such as:

CREATE UNIQUE INDEX idx_lower_unique 
   ON your_table (lower(the_column));

It would be great if this feature could be added.

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Comment migrated from Redmine: https://redmine.postgresql.org/issues/4260#note-1
Originally created by Khushboo Vashi at 2019-05-17 10:20:50 UTC.

Redmine ticket header update:

Name	Old Value	New Value
Sprint changed		Ready

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Comment migrated from Redmine: https://redmine.postgresql.org/issues/4260#note-2
Originally created by Aditya Toshniwal at 2019-06-05 06:01:44 UTC.

Redmine ticket header update:

Name	Old Value	New Value
Assigned To changed	Aditya Toshniwal

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Comment migrated from Redmine: https://redmine.postgresql.org/issues/4260#note-3
Originally created by Rahul Shirsat at 2021-06-25 11:15:09 UTC.

Redmine ticket header update:

Name	Old Value	New Value
Difficulty changed		Medium

[FaharAbbasRizvi]

This is resolved in the latest snapshot build and tested in all supported platforms.
https://www.postgresql.org/ftp/pgadmin/pgadmin4/snapshots/2023-08-07/