feat: support string hlc

[Jei-sKappa]

Closes #20.

---

Notes:
I also started working on adding a test case to sqlite_crdt.
Here’s what I’ve done so far:

    test('Merge changeset from other crdt', () async {
      final otherCrdt = await SqliteCrdt.openInMemory();
      await otherCrdt.execute('''
        CREATE TABLE users (
          id INTEGER NOT NULL,
          name TEXT,
          PRIMARY KEY (id)
        )
      ''');
      await insertUser(otherCrdt, 1, 'John Doe');

      final hlc = (await otherCrdt.query('SELECT * FROM users')).first['hlc'];

      final changeset = await otherCrdt.getChangeset();
      expect(changeset['users']!.first['id'], 1);
      expect(changeset['users']!.first['name'], 'John Doe');
      expect(changeset['users']!.first['hlc'], isA<String>());

      await crdt.merge(changeset);
      final result = await crdt.query('SELECT * FROM users');
      expect(result.first['name'], 'John Doe');
      expect(result.first['hlc'], hlc);
    });

However, when running this (with the updated crdt package), I get the following error:

  Unsupported operation: read-only
  package:sqflite_common/src/collection_utils.dart 156:5  QueryRow.[]=
  package:sql_crdt/src/sql_crdt.dart 165:17               SqlCrdt.merge.<fn>
  package:sqlite_crdt/src/sqlite_api.dart 32:12           ExecutorApi.executeBatch
  package:sql_crdt/src/sql_crdt.dart 155:15               SqlCrdt.merge
  ===== asynchronous gap ===========================
  test/sql_crdt_test.dart 187:7                           runSqlCrdtTests.<fn>.<fn>
  
  Bad state: No element
  dart:collection                 ListBase.first
  test/sql_crdt_test.dart 189:21  runSqlCrdtTests.<fn>.<fn>

This happens because the query method in the ExecutorApi class returns a QueryRow object, which is "immutable" and cannot be modified.

I came up with two possible solutions:

1) Update ExecutorApi’s query method to copy the row maps before returning them

class ExecutorApi extends DatabaseApi {
  @override
  Future<List<Map<String, Object?>>> query(String sql, [List<Object?>? args]) =>
      _db.rawQuery(sql, args).then((r) => r.map((e) => Map<String, Object?>.from(e)).toList());
  //  _db.rawQuery(sql, args); // instead of this
}

This ensures the returned data is mutable.
I don’t think this is the cleanest solution, but it works.

2) Update SqlCrdt's merge method to copy the record before updating it:

Future<void> merge(CrdtChangeset changeset) async {
  /* ... */

  await _db.executeBatch((executor) async {
    for (final entry in changeset.entries) {
      final table = entry.key;
      final records = entry.value;
      final keys = tableKeys[table]!.join(', ');

      for (final r in records) { // previously: for (final record in records) {
        final record = Map.from(r); // <-- add this line

        // Extract node id from the record's hlc
        record['node_id'] = (record['hlc'] is String // <-- This is causing the main issue
                ? (record['hlc'] as String).toHlc
                : (record['hlc'] as Hlc))
            .nodeId;
        // Ensure hlc and modified are strings
        record['hlc'] = record['hlc'].toString();
        record['modified'] = hlc.toString();

        /* ... */
      }
    }
  });

  /* ... */
}

---

Let me know which approach you prefer.
Also, should I file a separate issue for this problem?

[Jei-sKappa]

Just to clarify - this isn’t a regression from this PR, but rather something we didn’t have a test for yet: merging between two sqlite_crdt instances.