Deprecate callback hooks on_init_start and on_init_end

[daniellepintz]

What does this PR do?

Fixes #10894

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[tchaton]

LGTM !

[codecov]

Codecov Report

Merging #10940 (8da7733) into master (aeb0b55) will increase coverage by 32%.
The diff coverage is 100%.

❗ Current head 8da7733 differs from pull request most recent head 5158264. Consider uploading reports for the commit 5158264 to get more accurate results

@@            Coverage Diff            @@
##           master   #10940     +/-   ##
=========================================
+ Coverage      56%      88%    +32%     
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  Files         177      177             
  Lines       16545    16526     -19     
=========================================
+ Hits         9340    14548   +5208     
+ Misses       7205     1978   -5227