Track architectural improvements: Move code to core package

[coderabbitai]

Several TODOs in the codebase indicate planned architectural improvements to move code to a "core" package.

Found in:

• packages/starknet/lib/src/signer/base_signer.dart
• packages/starknet_provider/lib/src/model/function_call.dart

This issue tracks the planned architectural improvements to better organize the codebase by moving appropriate code to a core package.

Related PR: #445

[ptisserand]

Depends on #454