Allow to define const(s) to database

[apz28]

The reasons for this feature is to avoid re-declare them through out stored-procedures, triggers, scripts and also can be used in insert/select/update statements

The syntax should be similar to declare a variable such as below. The security is same as if user can create a stored-procedures...
SET CONST const-name = xyz...

const-name name syntax is identifiers but must be allowed for longer than 31 characters
All const-name must be unique, if later re-declared with same name -> error
It can be searched & used in similar of a variable but lower priority to variable/field

Also need to support a way to change the value
ALTER CONST const-name = xyz2...

[aafemt]

If your syntax isn't backed by SQL standard, I would suggest to use DECLARE CONSTANT <name> <datatype> = <value> instead to match declaration of a variable and other "declarable" objects.

[sim1984]

This will be in the form of package constants anyway. See #8916