Add hw wallet sign message

[moisses89]

Description

Add support to sign messages using hardware wallet on transaction service mode.
Close #354

Other changes

• sign_message just sign with the first provided owner because Safe Transaction Service is not accepting more than one signature to create the message. https://github.com/safe-global/safe-transaction-service/blob/49c65ec7187a20e06f466ecaeccbc601e8e0f547/safe_transaction_service/safe_messages/serializers.py#L34

[coveralls]

coverage: 87.131% (+0.2%) from 86.934%
when pulling 6670730 on add_hw_wallet_sign_message
into 88ca917 on main.

[coveralls]

coverage: 87.559% (+0.09%) from 87.47%
when pulling 3bcfc8f on add_hw_wallet_sign_message
into ddd66d6 on main.

[coveralls]

coverage: 87.559% (+0.09%) from 87.47%
when pulling 909e0f1 on add_hw_wallet_sign_message
into ddd66d6 on main.

[coveralls]

coverage: 88.571% (+0.07%) from 88.505%
when pulling a12b5e1 on add_hw_wallet_sign_message
into e829cd6 on main.

[falvaradorodriguez]

The changes in the source code are fine for me. In any case I validate it based on the tests (I can't test it without HW wallet).