tezos-codec: Optionally link to protocols
Context
Like we already do for the client and the node, when installing the software by opam. it is nice to be able to set old protocols of mainnet as optional dependencies so that poeple having them installed have support for these protocols in their tool but newcomers only install the "active" protocols which should be the only one they need under normal circumstances.
Manually testing the MR
As the protocols are in the codebase, they are available and therefore it changes nothing when you make, you still have all the protocols linked. You can check that by ./tezos-codec list encodings
In order to see a difference you need to scripts/opam-pin.sh then to opam install tezos-codec which should install only alpha protocol (rational below) and then install an other protocol and check that first opam recompiles tezos-codec and second tezos codec now knows about the encodings of that protocol
automatic testing
is done by the opam phase of the CI. Alpha is a non optional dep so that we ensure it links correctly when installed via opam.