Add add_custom_fee() and add_fee_exempt_key() to TopicUpdateTransaction

[exploreriii]

https://docs.hedera.com/native/consensus/update-topic

TopicUpdateTransaction provides set_custom_fees()/clear_custom_fees() and set_fee_exempt_keys()/clear_fee_exempt_keys(), but is missing the add versions:

• add_custom_fee()
• add_fee_exempt_key()

Such methods exist in the JS SDK
https://github.com/hiero-ledger/hiero-sdk-js/blob/main/src/topic/TopicUpdateTransaction.js

set_custom_fees(fees) — replace the pending list wholesale
add_custom_fee(fee) — append one to the pending list, creating it if unset

The author should study the reference implementations to understand style and conventions

[github-actions]

Thanks for your submission.

This issue is awaiting approval from @hiero-ledger/hiero-sdk-python-triage and @hiero-ledger/hiero-sdk-python-committers and @hiero-ledger/hiero-sdk-python-maintainers.

[manishdait]

Closing this a Duplicate we have #2447 and #2448