feat: Offline multi node signing support

[0xivanov]

Description:
The SDK currently does not support the use case of multiple signatures for multiple nodes when signing is done offline. Transaction.sign(key) works for multi node, multi sig but not offline. PrivateKey.signTransaction(transaction) supports offline multi sig, but not multi node.
This PR adds 2 new functions:

1. GetSignableBodyBytes: returns a list of SignableBody objects for each signed transaction in the transaction list.
2. AddSignatureV2: adds a signature to the transaction for a specific transaction id and node id.
   This is useful for signing chuncked transactions like FileAppendTransaction, since they can have multiple transaction ids.

Proposal reference https://github.com/hiero-ledger/sdk-collaboration-hub/blob/main/proposals/manual-signature-hsm-design-proposal.md

Related issue(s):

Fixes #1050 #1381

Notes for reviewer:

Checklist

• Documented (Code comments, README, etc.)
• Tested (unit, integration, etc.)

[lfdt-bot]

🎉 Snyk checks have passed. No issues have been found so far.

✅ security/snyk check is complete. No issues have been found. (View Details)

✅ license/snyk check is complete. No issues have been found. (View Details)

[codecov]

Codecov Report

Attention: Patch coverage is 84.70588% with 13 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
sdk/transaction.go	86.36%	6 Missing and 3 partials ⚠️
sdk/transaction_id.go	77.77%	3 Missing and 1 partial ⚠️

Files with missing lines	Coverage Δ
sdk/crypto.go	72.75% <100.00%> (+0.04%)	⬆️
sdk/transaction_id.go	58.08% <77.77%> (+2.37%)	⬆️
sdk/transaction.go	69.26% <86.36%> (+0.81%)	⬆️

... and 8 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.