Return Mempool @ given Stacks canonical chain block height

[gregorycoppola]

Is your feature request related to a problem? Please describe.
We do not have a Hiro-controlled way to look back at the size of the mempool at a given point in time.

To follow the Steady Lads mint, I made a custom node script to ping the API every 30 seconds, and get the size of the mempool at that time.

I can log onto https://stacksdata.info/#mempool and get very limited historical data, but neither the detail nor the data longevity are under our control.

I'd like a deployed way to be able to get the mempool size and contents over time.

Describe the solution you'd like
Store with each mempool transaction the Stacks chain canonical height. Then, we can query for that later, using either the API or the backend.

Describe alternatives you've considered
We have considered deploying an additional system to capture this kind of information but there was huge push back on a new system. I think the easiest could be to expand the API system.

We can add prometheus metrics to the stacks-node, and then record these over time, and I will add an issue about that. But, that won't have detailed information, the only thing we can add there is scalars.