Serialise the output of `grapher://grapher` steps to disk

[larsyencken]

Motivation

We would like to have alignment between our indicator and ETL APIs, however the flattening of dimensions causes a misalignment and means that there is nowhere in the ETL APIs that can give you the same data that's in a chart.

Proposal

We should serialise and publish the generated data frames for grapher://grapher steps, as well as shipping that data to MySQL. It might need a new channel.

[larsyencken]

/cc @danyx23 @Marigold @pabloarosado

[pabloarosado]

Thanks @larsyencken, just a quick note: By construction, the reason data://grapher steps exist (and that we don't publish garden steps directly) is to adapt our curated data to the neds of our grapher tool. So, if data://grapher are actually different from grapher://grapher steps, then they are not really fulfilling that promise, and therefore are a bit misleading (or redundant). The data team does not need to be aware of the technical difference between data://grapher and grapher://grapher steps. In fact, we usually just speak of "grapher steps". However, with the current implementation, data managers need to know that, for some reason, mdim steps depend on grapher://grapher steps (instead of the usual data://grapher). In my view, the ideal solution would be to have just one kind of grapher step (data://grapher). And grapher://grapher steps would simply be an implementation helper that data managers don't even need to be aware of.