Add Zcash plugin extension to BTCPayServer

Hi there,

We've been trying to figure out how to accept shielded ZEC through our donation infrastructure for a while. There is now a functional BTCPayServer plugin for accepting shielded Zcash: https://github.com/btcpay-zcash/btcpayserver-zcash-plugin/blob/master/docs/installation.md. (See also https://forum.zcashcommunity.com/t/nymvpn-now-accepts-shielded-zec-payments/51628).

Is there any reason I can't add this plugin to our BTCPayServer implementation and fiddle with and it to see if it does what we need?

Happy to wait for TPA to investigate. Also, this is not an emergency.

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