add license as a possible field for opam list

[raphael-proust]

Please update master_changes.md file with your changes.

[raphael-proust]

couldn't run the tests locally (i think it requires some kind of set up) but I did run the compiled binary with the new flag and it worked

[kit-ty-kate]

Thanks for your contribution. Would you have time to add some tests? (format is documented in https://github.com/ocaml/opam/blob/master/tests/reftests/readme.md) If not we'll do it whenever we have some time

[raphael-proust]

I had forgotten to push the test file, doing so now

[kit-ty-kate]

Looking at this change once again i'm realizing that it is already possible print the exact same thing using the raw field syntax:

opam list --columns=name,license:

This isn't ideal and has been discussed in the past (#3664). We could at least document it in the man section for --columns but this should also be improved at some point imo (although i'm not sure what's the best way to do that in a backward-compatible manner, if you have ideas feel free to drop them here)

[raphael-proust]

I didn't know about the raw-field syntax. It should be documented in the man/help I think. I can add that.

Also the inconsistency between columns and field in show/list is surprising. But that's for another PR I guess.

[raphael-proust]

Added a specific mention in the doc. I realised it was mentioned before but this one is more visible.

[kit-ty-kate]

modulo the PR title that needs to be changed, lgtm

[kit-ty-kate]

Suggested change

	colon, e.g., $(b,license:)) selects an arbitraty field. \
	colon, e.g., $(b,license:)) selects an arbitrary field. \

[kit-ty-kate]

ah my bad i looked only at the new changes, the rest of the PR should probably require a longer discussion. The master_changes.md entries should also be updated (doc + test). I can do the last rebase at the end if needed

[rjbou]

It worth also adding a full test for opam list, but that's for another PR too.

[avsm]

Nice! Just for the record, this is what moby/vpnkit (from Docker) uses to autogenerate the licenses we ship with Docker; https://github.com/moby/vpnkit/blob/master/dune#L12 -- the raw field syntax is fine, but we can upgrade to this once released.