Support SQL Server ALTER TABLE ADD DEFAULT constraints

[minleejae]

Fixes #2020

The CREATE TABLE example is already supported on current master. This PR addresses the remaining ALTER TABLE ADD DEFAULT constraint syntax.

Changes

• Parse ADD [CONSTRAINT name] DEFAULT expression FOR column [WITH VALUES].
• Represent the default as DefaultConstraint, a NamedConstraint with a structured value expression and target Column, exposed through AlterExpression.getIndex().
• Reuse table-element traversal/rendering and share DEFAULT rendering between AST and deparser. Custom expression deparsers can rewrite both the default value and target column.
• Integrate expression and metadata validation, including unnamed defaults and checking a named constraint as a new constraint rather than an existing index.

Existing inline column defaults and ALTER COLUMN SET/DROP DEFAULT remain unchanged. This does not attempt database-level validation of which expressions SQL Server permits as defaults.

Verification

• Full Gradle check (tests, Checkstyle, PMD, Spotless).
• Maven clean verify with Java 17.
• The issue's ALTER TABLE example, quoted identifiers, nested parentheses, functions, sequence defaults, multiple actions, negative syntax, AST mutation, custom deparsing, and metadata validation.

Reference: SQL Server ALTER TABLE table constraints.