Change message format when including node as delegate to Staking Safe

[QYuQianchen]

To add a node as a delegate to a Safe, it requires a signature from a Safe owner via POST /v1/delegates/ endpoint (see https://safe-transaction-mainnet.safe.global/). The message to be signed is currently only a concatenated string and thus not compatible to EIP-712.
Changes need to be introduced:

• Introduce changes in hoprnet/safe-transaction-service link to accept EIP712
• Update StakingHub so that the "add delegate" step prepares a EIP712-compatible message and asks for admin signature

[mjadach-iv]

Issue was reported to Safe too, and they said they will implement it:
safe-global/safe-transaction-service#1857 (comment)

[mjadach-iv]

Seems we can work on it now, need to verify with our safe infra