Add project-based "product" attribute

[durack1]

As has now been discussed in a couple of threads across repos, the product attribute has been used to delineate between model output in CMIPx model-output (e.g. `CMIP6_Amon.json), and various flavours of observational, derived and other data across input4MIPs and obs4MIPs (see discussion in PCMDI/cmor#723 (comment) and WCRP-CMIP/CMIP6Plus_CVs#45.

To remove this from the table headers, so table files can be used across projects, this will need to be registered in the Project_CVs, with a look-up validation entry CV is required in this repo to catch invalid use cases.

As a quick pass and to account for all existing examples, we'd need something like:

{
  "product": {
    "model-output",  # CMIP6, CORDEX-CMIP6; we also have output, output, output1 and output2 in CMIP5
    "observations",  # input4MIPs, obs4MIPs
    "reanalysis",  # input4MIPs, obs4MIPs
    "forcing",  # input4MIPs
    "derived",   # input4MIPs, obs4MIPs
    ...

@taylor13 @mauzey1 @matthew-mizielinski @wolfiex ping