Allow multiple trusted forwarders in Boson Voucher

[zajck]

BosonVoucher currently uses OZ's ERC2771ContextUpgradeable to handle metatransactions. It currently supports only 1 trusted forwarder address, which is set during the deployment.

If we want to change trusted forwarder address, currently the only way is to redeploy voucher contract with new constructor argument. Furthermore, we might want to support multiple forwarders.

ToDo:

• change immutable trusted forwarder with storage variable (either mapping or array)
• add methods to manage trusted forwarders (add/remove)

Since boson vouchers use beacon proxy patter, list of trusted forwarders cannot be stored in the vouchers itself (each update would require to update each cloned instance individually). Possible implementations:

• voucher contract calls isTrustedForwarder on beacon contract
• we deploy another forwarder between trusted forwarder and boson vouchers. Vouchers then always accept calls only from the second forwarder. And this second forwarder can accept calls from multiple trusted forwarders and implements logic to manage them.