[feature]: `peer_pubkey` in `AddInvoiceRequest` should accept an array

[ZZiigguurraatt]

peer_pubkey in AddInvoiceRequest should accept an array. Right now we only have a choice of leaving the value empty to select all peers with a given asset_id or a single peer to do an RFQ with and add to the route hints of an invoice. We should be able to select multiple peers, but not necessarily all peers.

[ZZiigguurraatt]

We have this same issue for SendPayment in #1613

taproot-assets/taprpc/tapchannelrpc/tapchannel.proto

Lines 125 to 130 in aa55b42

	// The node identity public key of the peer to ask for a quote for sending
	// out the assets and converting them to satoshis. If set, only a quote with
	// this peer may be negotiated to carry out the payment.
	bytes peer_pubkey = 3;

and the docs are incorrect.