* Dowload Row Limit Env Var

Adds `MB_DOWNLOAD_ROW_LIMIT` to enable changing the row limit on downloads and alert/subscription attachments (not the
rendered tables, but the .csv, etc.).

Based on: #44982 (Thanks, @r-kot)

Partially implements: #28144

The difference in this PR compared to #44982 is that the download limit applies to all downloads, the only exception
being when the limit is above `qp.i/absolute-max-results` (1048575, based on Excel's limitation); in such a case, the
user supplied limit is only used if the download is csv or json, and `qp.i/absolute-max-results` is used for xlsx.

This PR also fixes alert/subscription attachment limits; prior to this, they were set to the in-app limit of 2000
rows, but now they will follow the user supplied download-row-limit.

This PR also adds a test to the downloads-and-exports test namespace, confirming that they follow the supplied limit,
or the max limit if none is supplied.

* add test confirming the default limit works

* fix test to use the download-row-limit

* address review feedback

* Update src/metabase/public_settings.clj

Co-authored-by: Cal Herries <39073188+calherries@users.noreply.github.com>

* add test covering case where download-row-limit is unset

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Co-authored-by: Cal Herries <39073188+calherries@users.noreply.github.com>