Add option to specify content header length

[benoit74]

Fix #320

To be reviewed / merged after #319

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 84.61%. Comparing base (8f79546) to head (f8fe4bc).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #321 +/- ## ========================================== + Coverage 84.57% 84.61% +0.03% ========================================== Files 14 14 Lines 1245 1248 +3 Branches 246 246 ========================================== + Hits 1053 1056 +3 Misses 149 149 Partials 43 43 ```

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[benoit74]

It's definitely kind of an expert value. But needs to be exposed in the Zimfarm so that it can be tweaked when needed.