feat(dashboards): put in backend validation for heat map request fields

[nikkikapadia]

Closes DAIN-1719

Just adds the validation we have for heatmaps in the frontend to the backend as well. This is only for dashboards right now, i can add in the saved queries one in another PR.

basically we are following the following conditions:

• heat maps for trace metrics only (this was already previously implemented)
• no equations
• singular queries only
• one aggregate
• must use distribution metrics
• no group-bys
• no thresholds

[linear-code]

DAIN-1719

[cursor]

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