refactor(pkg): add EncodeToUpperHex utility and use across packages

[kpitapeersyst]

refactor(pkg): add EncodeToUpperHex utility and use across packages

Description

This PR centralizes the scattered hex.EncodeToString + strings.ToUpper pattern across the codebase into a single EncodeToUpperHex utility.

Type of change

• Bug fix
• New feature
• Breaking change
• Documentation update
• Refactoring

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code where needed
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective
• New and existing unit tests pass locally with my changes

Changes

pkg

• Added EncodeToUpperHex utility in pkg/hexutil and replaced all scattered hex.EncodeToString + strings.ToUpper calls across the codebase

xrpl

• Used existing EncodeToHashString helper in tx hash computation

Notes

fmt.Sprintf("%X", b) is more concise but as it uses reflection, it's slower, so we keep strings.ToUpper(hex.EncodeToString(b)).