refactor: Move source go files in /sdk package

[0xivanov]

Description

This PR reorganizes the project structure by introducing an sdk directory as the primary location for SDK-related functionality.

Changes Made

1. Created an sdk directory to serve as the central location for SDK source files.
2. Moved all existing source files into the sdk directory.

Why Not Sub-Packages?

• Sub-packages are not introduced in this PR to keep the structure simple and minimize disruption to the current development workflow.
• Sub-packaging will be considered in future iterations as the codebase grows, especially if functionality becomes more modular and requires logical grouping.

Why /sdk Instead of /src?

• Avoiding Non-Idiomatic Practices: Other popular Go libraries, such as aws-sdk-go and go-redis, avoid using /src and instead organize their files directly in meaningful directories like sdk, pkg, or at the root level.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Files with missing lines	Coverage Δ
sdk/abi.go	59.70% <ø> (ø)
sdk/abi_decode.go	54.09% <ø> (ø)
sdk/abi_encode.go	47.58% <ø> (ø)
sdk/abi_type.go	78.19% <ø> (ø)
sdk/account_allowance_approve_transaction.go	72.46% <ø> (ø)
sdk/account_allowance_delete_transaction.go	90.35% <ø> (ø)
sdk/account_balance.go	100.00% <ø> (ø)
sdk/account_balance_query.go	85.84% <ø> (ø)
sdk/account_create_transaction.go	97.25% <ø> (ø)
sdk/account_delete_transaction.go	97.59% <ø> (ø)
... and 114 more

... and 120 files with indirect coverage changes