mod+internal: copy MuSig2 v0.4.0 code from btcd/btcec/v2

With this commit we copy the exact code of the MuSig2 code as found in
github.com/btcsuite/btcec/v2/schnorr/musig2 at the tag btcec/v2.2.2.
This corresponds to the MuSig2 BIP specification version of v0.4.0.

go.mod

@@ -16,6 +16,7 @@ require (
 	github.com/btcsuite/btcwallet/wtxmgr v1.5.0
 	github.com/coreos/go-systemd v0.0.0-20190719114852-fd7a80b32e1f
 	github.com/davecgh/go-spew v1.1.1
+	github.com/decred/dcrd/dcrec/secp256k1/v4 v4.0.1
 	github.com/go-errors/errors v1.0.1
 	github.com/golang/protobuf v1.5.2
 	github.com/gorilla/websocket v1.4.2
@@ -76,7 +77,6 @@ require (
 	github.com/coreos/go-systemd/v22 v22.3.2 // indirect
 	github.com/cpuguy83/go-md2man/v2 v2.0.0 // indirect
 	github.com/decred/dcrd/crypto/blake256 v1.0.0 // indirect
-	github.com/decred/dcrd/dcrec/secp256k1/v4 v4.0.1 // indirect
 	github.com/decred/dcrd/lru v1.0.0 // indirect
 	github.com/dsnet/compress v0.0.1 // indirect
 	github.com/dustin/go-humanize v1.0.0 // indirect