Add ability to diff envs locally

[harsimranmaan]

qbec diff env has been super helpful to see changes for an env before apply. It would also be helpful to be able to two env configs locally. This would help with refactoring, catching issues like missing config overrides, etc. Something like qbec diff env1 env2. There might be value in doing this for remote envs too but I don't have a pressing use case for that yet.

[harsimranmaan]

diff <(qbec show env1) <(qbec show env2) works as a workaround.

[gotwarlost]

also qbec component [-O] diff and qbec params diff exist