Feat cross platform support

[abenso]

• Add cross-platform buffer support for browser environments using buffer/ polyfill
• Introduce toBuffer() utility function that converts multiple input types (Buffer, Uint8Array, or hex string)
  to Buffer
• Add new flexible input types: TransactionBlobInput and TransactionMetadataBlobInput
• Update all signing and metadata methods (sign, signEd25519, signEcdsa, signRaw, getTxMetadata) to accept the
  new flexible input types
• Add comprehensive unit tests for the toBuffer function
• Update dependencies

This change improves developer experience by allowing transaction blobs to be passed as hex strings or
Uint8Array in addition to Buffer, making the library easier to use in browser environments where Buffer is not
natively available.

This is a fully backward-compatible change - existing code using Buffer will continue to work without any
modifications. The new input types are additions, not replacement

Close #141

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