feat: improve BinaryWriter API

[TheOneWithTheBraid]

• extend default buffer size (thanks to @nico-famedly)
• allow HiveCipher to asynchronously perform crypto
• implement HiveAesThreadedCipher for Hive Flutter
• fix missing file in .gitignore

In theory, no public API should be made incompatible here as HIveCipher
was simply converted from T Function() to FutureOr<T> Function().

The idea behind the asynchronous operation is

a) multithreading using e.g. the compute() function in Flutter or
b) platform-native implementations making use of hardware-accelerated
cryptographic implementations.

CC: @nico-famedly maybe you like to try out this?

Signed-off-by: TheOneWithTheBraid the-one@with-the-braid.cf

[TheOneWithTheBraid]

@nico-famedly: Can you try out the HiveAesNativeCipher()?

[TheOneWithTheBraid]

I parallelized the binary writer API in a way that the hive cipher now only needs to immediately return the pretended length of the encrypted data while the actual encryption can happen in background - also several crypto operations at once.

That's what the _pendingOperations cache is for.

When the native implementation is told to write to disk (e.g. by flush() or close()), the FutureOr<Uint8List> toBytes() is called.

There, the binary writer checks whether there are running operations in background and correspondingly waits for them to complete. Afterwards, the buffer is returned.

https://github.com/hivedb/hive/pull/986/files#diff-86c49de9691825b1989a3d306e00d9b3e270eec23690e7e4c5d6236f11ad5ac9R455

For some reason, at this place, the returned buffer is always an empty buffer filled with 0 only, even though

https://github.com/hivedb/hive/pull/986/files#diff-86c49de9691825b1989a3d306e00d9b3e270eec23690e7e4c5d6236f11ad5ac9R435

exactly before, a print of the buffer shows the correct bytes inside.

https://github.com/hivedb/hive/pull/986/files#diff-86c49de9691825b1989a3d306e00d9b3e270eec23690e7e4c5d6236f11ad5ac9R456

Also, this here ensures the cache of pending operations is empty before returning the buffer.

Any idea?