Identify variable sponsor mip table era

[durack1]

It would be useful to add a provenance entry that identifies the mip_era/cmipX-cmor-tables version in which a variable was first defined

mip_era	cmor version	url
amip1	NA	https://pcmdi.llnl.gov/mips/amip/OUTPUT/WGNEDIAGS/
amip2	NA	https://pcmdi.llnl.gov/mips/amip/OUTPUT/AMIP2/outlist.html
cmip1	NA
cmip2	NA
cmip3	1.0	https://github.com/PCMDI/cmip3-cmor-tables/tree/master/Tables
cmip5	2.x	https://github.com/PCMDI/cmip5-cmor-tables/tree/master/Tables
cmip6	3.x	https://github.com/PCMDI/cmip6-cmor-tables/tree/master/Tables
cmip6plus	4.x	https://github.com/PCMDI/mip-cmor-tables

There may be some provenance flags in the CMIP6 Data Request that provide further context, particularly when it comes to CMIP6 activity requested new variables e.g. prov[i]: Amon ((isd.003)) ; provNote[i]: Amon_2d

[durack1]

Not sure if Milestone 6.6 makes sense here, but just tagging as provenance would be great to complete under this milestone