Populating json colorbar file with environmental variables

Currently, json entries are fixed for specific variables (where cmap is defined, along with min/max value for colorbar). Some diagnostics much need adaptive colorbar ranges, such as the age of air diagnostic. This colorbar will extend from 0 to the length of the forecast, defined as $CSET_ANALYSIS_PERIOD. Is there a way we can use $CSET_ANALYSIS_PERIOD in the json and when reading this in the plot operator, it fills this with the appropriate environment value?

also addressed here: #1139

Start by tracing the plot operator's JSON colorbar reader and how it currently handles fixed cmap/min/max entries; inspect where CSET_ANALYSIS_PERIOD is available in the runtime environment. Done means an environment reference in the JSON resolves to the forecast length for adaptive colorbar ranges while existing fixed ranges continue to work.

Written by the indexing model from the issue text.