btcutil: consider adding a new method that verifies address is for the correct net

[torkelrogstad]

In its current form, btcutil.DecodeAddress has some slightly surprising behavior:

addr, err := btcutil.DecodeAddress("bc1q7cyrfmck2ffu2ud3rn5l5a8yv6f0chkp0zpemf", &chaincfg.TestNet3Params)
if err != nil {
	log.Fatal(err)
}
log.Println("address: ", addr.String())

This doesn't error, but instead decoded just fine. It's explained in the godoc that the net is only used for legacy addresses. However, it looks like this is often overlooked. One example would be the instances of this in lnd found in this PR: lightningnetwork/lnd#6448

Would it be an idea to add a new method that decodes the address, and then verifies it is for the given network? Names could for example be DecodeAddressVerify, or DecodeAddressForNet.

[guggero]

I like the idea of adding DecodeAddressForNet. Will review a PR. Would be nice to add a positive/negative unit test for both the existing and the new functions.

[Roasbeef]

Historical context: btcsuite/btcutil#133