Changing multiple columns of composite primary key is not supported

[kvch]

I added a new test TestAlterPrimaryKeyColumns with multiple test cases in #930.
However, we do not yet support altering multiple columns in the same composite key.

A test that fails:

         {
             name: "alter a composite primary key: two column changes type",
             migrations: []migrations.Migration{

            {
                Name: "01_create_table",
                Operations: migrations.Operations{
                    &migrations.OpCreateTable{
                        Name: "people",
                        Columns: []migrations.Column{
                            {
                                Name: "id1",
                                Type: "int",
                                Pk:   true,
                            },
                            {
                                Name: "id2",
                                Type: "int",
                                Pk:   true,
                            },
                            {
                                Name:     "name",
                                Type:     "varchar(255)",
                                Nullable: true,
                            },
                        },
                    },
                },
            },
            {
                Name: "02_alter_column",
                Operations: migrations.Operations{
                    &migrations.OpAlterColumn{
                        Table:  "people",
                        Column: "id1",
                        Type:   ptr("bigint"),
                        Up:     "CAST(id1 AS bigint)",
                        Down:   "SELECT CASE WHEN id1 < 2147483647 THEN CAST(id1 AS int) ELSE 0 END",
                    },
                    &migrations.OpAlterColumn{
                        Table:  "people",
                        Column: "id2",
                        Type:   ptr("bigint"),
                        Up:     "CAST(id2 AS bigint)",
                        Down:   "SELECT CASE WHEN id2 < 2147483647 THEN CAST(id2 AS int) ELSE 0 END",
                    },
                },
            },
        },
        afterStart: func(t *testing.T, db *sql.DB, schema string) {
            bigint := "31474836471" // A value larger than int can hold
            integer := "100"

            // Inserting a row with integer id into the old schema should succeed
            MustInsert(t, db, schema, "01_create_table", "people", map[string]string{
                "id1":  integer,
                "id2":  integer,
                "name": "alice",
            })
            // Inserting a row with integer bigint id into the old schema should fail
            MustNotInsert(t, db, schema, "01_create_table", "people", map[string]string{
                "id1":  bigint,
                "id2":  integer,
                "name": "alice",
            }, testutils.NumericValueOutOfRangeErrorCode)

            // Inserting a row with bigint id into the new schema should succeed
            MustInsert(t, db, schema, "02_alter_column", "people", map[string]string{
                "id1":  bigint,
                "id2":  bigint,
                "name": "alice",
            })
        },
        afterRollback: func(t *testing.T, db *sql.DB, schema string) {
        },
        afterComplete: func(t *testing.T, db *sql.DB, schema string) {
            // Inserting a row with integer bigint into the new schema should succeed
            MustInsert(t, db, schema, "02_alter_column", "people", map[string]string{
                "id1":  "31474836472",
                "id2":  "31474836472",
                "name": "bob",
            })
        },
    },