## Environment
**Liquibase Integration & Version**: spring boot 2.4.2

**Database Vendor & Version**: postgresql

**Operating System Type & Version**: all

## Description
The [documentation ](https://docs.liquibase.com/change-types/community/add-primary-key.html) states that clustered is supported by all dbms. 

Not all databases support clustered primary keys, but Postgresql is one that does. The code to set the primary key as clustered exists in addPrimaryKey, but the validation check blocks it from being run. The validation also fails if clustered is set to "false".

## Steps To Reproduce
```yaml
- changeSet:
    id: 1-pkey
    author: me
    changes:
    - addPrimaryKey:
        tableName: testTable
        columnNames: id
        constraintName: testTable_pkey
        clustered: false
```

## Actual Behavior
```
liquibase.exception.ValidationFailedException: Validation Failed:
     1 changes have validation failures
          clustered is not allowed on postgresql [...]
```

## Expected/Desired Behavior
* Running 'liquibase update' should run without failure and create the primary key as clustered or not depending on the clustered argument


validationErrors.checkRequiredField("tableName", addPrimaryKeyStatement.getTableName());
        if (!((database instanceof MSSQLDatabase) || (database instanceof MockDatabase))) {
            if ((addPrimaryKeyStatement.isClustered() != null) && !addPrimaryKeyStatement.isClustered()) {
                validationErrors.checkDisallowedField("clustered", addPrimaryKeyStatement.isClustered(), database);
            }
        }
        if (!((database instanceof OracleDatabase) || (database instanceof AbstractDb2Database))) {
            validationErrors.checkDisallowedField("forIndexName", addPrimaryKeyStatement.getForIndexName(), database);
        }
        return validationErrors;
    }
    @Override
    public Sql[] generateSql(AddPrimaryKeyStatement statement, Database database, SqlGeneratorChain sqlGeneratorChain) {
        String sql;
        if ((statement.getConstraintName() == null) || (database instanceof MySQLDatabase) || (database instanceof
            SybaseASADatabase)) {
            sql = "ALTER TABLE " + database.escapeTableName(statement.getCatalogName(), statement.getSchemaName(), statement.getTableName()) + " ADD PRIMARY KEY (" + database.escapeColumnNameList(statement.getColumnNames()) + ")";
        } else {
            sql = "ALTER TABLE " + database.escapeTableName(statement.getCatalogName(), statement.getSchemaName(), statement.getTableName()) + " ADD CONSTRAINT " + database.escapeConstraintName(statement.getConstraintName())+" PRIMARY KEY";
            if ((database instanceof MSSQLDatabase) && (statement.isClustered() != null)) {
                if (statement.isClustered()) {
                    sql += " CLUSTERED";
                } else {
                    sql += " NONCLUSTERED";
                }
            }
            sql += " (" + database.escapeColumnNameList(statement.getColumnNames()) + ")";
        }
        if ((StringUtil.trimToNull(statement.getTablespace()) != null) && database.supportsTablespaces()) {
            if (database instanceof MSSQLDatabase) {