Allow optional Polygon private key for read-only operations

[aurelticot]

Instantiating an account in "web3" mode requires a Polygon private key.

When we don't want to create or revoke DIDs and we are sure the account exists, we want to be able to use it for all operations not requiring writing on the blockchain, without passing the private key.

https://github.com/verida/verida-js/blob/504047289efd7b5d9792677724036cab9b98da1a/packages/did-client/src/did-client.ts#L72-L74

Should also check here: https://github.com/verida/verida-js/blob/504047289efd7b5d9792677724036cab9b98da1a/packages/vda-web3-client/src/VeridaContractBase.ts#L85-L90