[feature]: Replace RPC endpoints AddAssetSellOrder and AddAssetBuyOrder with InitRfq*

[ffranr]

We need to update our RPC interface before the 0.4.2 release to ensure stability and prepare for asset-to-asset channel payments.

The current buy/sell terminology doesn't accurately represent asset-to-asset payments, where we divest and receive tap assets in the same event. Using buy/sell terminology also causes unnecessary complications for developers; the new endpoint should have fields that directly relate to the invoice payment process or the invoice settlement receive process.

Therefore, we should replace the AddAssetSellOrder and AddAssetBuyOrder endpoints with a new InitRfq* endpoint that initiates the RFQ process when called.

[ffranr]

It's actually simpler with two new endpoints rather than one:

Replace AddAssetBuyOrder with InitRfqRecvInvoicePayment:

message InitRfqRecvInvoicePaymentRequest {
    // The asset that we wish to receive as payment to settle the invoice.
    AssetSpecifier payment_asset = 1;

    uint64 payment_asset_amount = 2;

    uint64 invoice_expiry = 3;

    bytes peer_pub_key = 4;

    uint32 rfq_timeout_seconds = 5;
}

Replace AddAssetSellOrder with InitRfqPayInvoice:

message InitRfqPayInvoiceRequest {
    // The asset which this (wallet) node will use to pay the invoice.
    AssetSpecifier payment_asset = 1;

    // units: msats
    uint64 invoice_amount = 2;
    
    // units: msats
    uint64 max_routing_fee = 3;

    // units: unix timestamp in seconds
    uint64 invoice_expiry = 4;

    bytes peer_pub_key = 5;

    uint32 rfq_timeout_seconds = 6;
}

[ffranr]

From standup: we're not going to prioritize this very highly. Probably wont be in the next release. We may add a comment to the existing endpoints to note that they may be revised soon.