Add run_constrained field linking aiida-core & services versions

[chrisjsewell]

As discussed in aiidateam/aiida-core#4147 (comment),
Conda is really not happy trying to solve the environment for aiida-core + aiida-core.services.

Maybe a root issue here is that aiida-core and aiida-core.services are two completely separate packages, when really you want services to act like extra requirements do in setup.py, e.g. aiida-core[services].
Having a quick google, it might be good to add a https://docs.conda.io/projects/conda-build/en/latest/resources/define-metadata.html#run-constrained in the aiida-core package, to tie it to the same version of aiida-core.services i.e.:

requirements:
  run_constrained:
    - aiida-core.services =={{ version }}

which would constrain the possibilities for conda environment solving

[ltalirz]

Sounds good to me!