Consider putting a mip table style flag on mapping plugins

We have a number of places where we have a "if CMIP6 style do ... else if CMIP7 style do ..." code block -- to avoid depending directly on a particular MIP era value we could add a style flag to the mappings plugins to handle this.

Start by locating the mapping plugins and the places that branch on CMIP6 versus CMIP7 style, then read how MIP era values are currently passed through them. Done means the relevant mappings expose a style flag and those conditional checks no longer depend directly on a particular MIP era value.

Written by the indexing model from the issue text.