Add the ability to copy configs between stacks

[stack72]

Fixes: #1583

Please note, I specifically tried to keep this code simple. As part of the work to change secrets provider, we will need to refactor some of this code around the building of a list of config to encrypt with a new provider

[justinvp]

Some questions and suggestions. I also want to take this for a spin myself.

Tests?

[justinvp]

Some more comments, but getting closer.

I'd also still love to see some tests. Perhaps add some for Map.Copy in sdk/go/common/resource/config/map_test.go (and Value.Copy, if you add that, to sdk/go/common/resource/config/value_test.go)?

For the tests, I could imagine writing a custom crypter amongst the tests like (untested):

// Crypter that just adds a prefix to the plaintext string when encrypting,
// and removes the prefix from the ciphertext when decrypting.
type prefixCrypter struct {
	prefix string
}

func newPrefixCrypter(prefix string) Crypter {
	return prefixCrypter{prefix: prefix}
}

func (c prefixCrypter) DecryptValue(ciphertext string) (string, error) {
	return strings.TrimPrefix(ciphertext, c.prefix), nil
}

func (c prefixCrypter) EncryptValue(plaintext string) (string, error) {
	return c.prefix + plaintext, nil
}

And then in tests, create a config map for the source stack with some secure values that use newPrefixCrypter("stackA"), and then copy to a "new stack" with newPrefixCrypter("stackB") and confirm the secure values in the result match as expected for "stackB".

[stack72]

Thanks for the review @justinvp - just working on the tests right now - this will be ready again for review soon

[justinvp]

Two more comments, otherwise LGTM! 🎉

[stack72]

Make the suggested changes and will merge on a successful build