Add translate_pk method for ConfidentialDescriptor

[Velnbur]

The reasoning behind this PR is a small enhancement in developer experience. miniscript::Descriptor has translate_pk method, but ConfidentialDescriptor does not. Because of this, to translate the inner descriptor, the user needs to extract it and then move the key:

let conf_descriptor = ConfidentialDescriptor::<DescriptorPublicKey>::from_str(&descriptor_str).unwrap();

let definite_desc = conf_descriptor
    .descriptor
    .translate_pk(&mut translator)
    .unwrap();

let definite_conf_descriptor = ConfidentialDescriptor {
    descriptor: definite_desc,
    key: conf_descriptor.key,
};

definite_conf_descriptor.address(SECP256K1, &AddressParams::LIQUID)

When in miniscript, this can be done with method chain calls:

let descriptor = Descriptor::<DescriptorPublicKey>::from_str(&wallet.descriptor).unwrap();

descriptor
    .translate_pk(&mut translator)
    .unwrap()
    .address(Network::Bitcoin)

[apoelstra]

ACK d12cf6c; successfully ran local tests; yeah, this seems useful, though I wish we had a more elegant way to treat blinding keys like descriptor keys

[apoelstra]

FYI I disabled the integration tests locally. I will PR to fix up CI and see if we can get a fix on the problem, but let's get this and #100 in first.