[YSQL] Support renaming constraints in ALTER TABLE

[ndeodhar]

Jira Link: DB-1574
Add support for ALTER TABLE name RENAME CONSTRAINT

[hiepd]

@ndeodhar Would like to look at this issue if it's still open. From the master issue, it seems that this is done. Thanks!

[ndeodhar]

Hi @hiepd , thanks for your interest in contributing! This is still available (there was a mistake in the master issue, corrected it now). Please assign this to yourself if you plan to pick this up.

Would be great if you can enable these two flavors:
ALTER TABLE tablename RENAME CONSTRAINT oldname TO newname
ALTER TABLE IF EXISTS tablename RENAME CONSTRAINT oldname TO newname

[devabhishekpal]

Hey... I wanted to work on this issue if I could be pointed to which file I need to start digging into

[lukaseder]

Note that when trying to run this statement, this issue isn't referenced yet as for other not yet supported syntax. Instead, the error message suggests creating a new issue:

SQL Error [0A000]: ERROR: ALTER TABLE RENAME CONSTRAINT not supported yet
  Hint: Please report the issue on https://github.com/YugaByte/yugabyte-db/issues
  Position: 1