FBC should be in their own repo to decorrelate them from release branching

We must also find a way to better handle nudging from operator (bundle) release branches.

E.g: nudging from operator-bundle:release-1.8 should affect only the 1.8 bundle referenced in FBC, and in parallel we should also accept nudging from operator-bundle:main to the "next" bundle referenced in FBC.

I think what we could do is to change this structure in `catalog` directory:

catalog
|-- released
|   |-- 1.0.0.yaml
|   |-- 1.1.0.yaml
|   \-- etc.
|-- index.yaml
\-- rc.yaml

into this:

catalog
|-- released
|   |-- 1.0.0.yaml
|   |-- 1.1.0.yaml
|   |-- etc.
|   \-- index.yaml         // last released index
\-- candidates
    |-- 1.8.0.yaml
    |-- 1.8.0-index.yaml   // index that adds 1.8
    |-- 1.8.0-bundle_digest.sh
    |-- main.yaml
    |-- main-index.yaml    // index that adds main
    \-- main-bundle_digest.sh

So we can have several candidates in parallel

But for nudging we still need to find a way to make them point to the correct xxx-bundle_digest.sh