Re-enable influxd generate command

## What

The `influxd generate` command was disabled during the TSM engine migration, as it was not compatible with InfluxDB 1.x data storage.

## How

The `pkg/data/gen` package was ported from InfluxDB 1.x. Forward port the remaining generate tooling components from 1.x to write to multiple shards and re-enable the `influxd generate` command.

Start by reading the pkg/data/gen package and locating the disabled influxd generate command. Compare the remaining tooling components with their InfluxDB 1.x counterparts, then verify that generation writes to multiple shards and that the command is enabled again.

Written by the indexing model from the issue text.