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.

verida-js/packages/did-client/src/did-client.ts

Lines 72 to 74 in 5040472

	if (callType == 'web3' && !web3Config.privateKey) {
	throw new Error('Web3 transactions must specify `web3config.privateKey`')
	}

Should also check here:

verida-js/packages/vda-web3-client/src/VeridaContractBase.ts

Lines 85 to 90 in 5040472

	if (this.signer === undefined) {
	if ( web3Config.privateKey )
	this.signer = new Wallet(web3Config.privateKey, this.contract.provider)
	else
	throw new Error('either Signer or privateKey is required to initialize')
	}