Remove Uint64List support from polkadart_scale_codec and polkadart_cli packages

[Lohann]

Context

Dart Web doesn't support Uint64List, and some packages rely in this data structure, we must remove Uint64List references otherwise polkadart_scale_codec cannot be used on the web. Change all ocurrences to lists of BigInt.

• [dart2js] why Uint64List does NOT use js' BigUint64Array, instead of throwing "Uint64List not supported on the web" dart-lang/sdk#50332

Acceptance Criteria

• Remove Uint64List and Int64List support from u64 and i64 codecs.
• Refactor polkadart_cli for not use U64Sequence, I64Sequence, U64Array and I64Array codecs.
• Unit tests passes
• Update CHANGELOG.md for both packages.

[avive]

Thank you for this lovely lib! We consider using it for Karma Coin mobile app. Can you guys please confirm that supporting web apps is on the project short-term intention to support web apps and fix this issue?

[Lohann]

@avive yes! the plan is fully support all platforms supported by flutter, currently the only blocker for web is this issue here, which will be fixed shortly.