Support `build/pin_depends`

[tsibley]

For certain kinds of meta-packages, the build/pin_depends option is really useful. It looks like Boa once had support with 95cab5400d54a54526988e7fbb8740e0f58d2015:

https://github.com/mamba-org/boa/blob/95cab5400d54a54526988e7fbb8740e0f58d2015/boa/cli/new_build.py#L598-L622

but it was removed not long after with bbc1713.

Current schema model notes that no usage was found:

https://github.com/mamba-org/boa/blob/b8fbbdf11118da414c56413879e5397c13daeaa2/boa/schemas/model.py#L172

but well, I'd like to use it, and I know the anaconda meta-package also uses it. :-)

[wolfv]

I checked the conda-forge recipes (https://cs.github.com/?scope=org%3Aconda-forge&scopeName=conda-forge&q=pin_depends+language%3Ayaml) and didn't find any usage. We can consider it if you send a PR with a test :)

[tsibley]

Hmm, ok. Out of publicly available code, I did find MOOSE using it.