Create temporary bgw function in _timescaledb_functions schema

[svenklemm]

Only creating functions/procedures in a dedicated schema allows us to further lockdown the _timescaledb_internal schema by removing EXECUTE privilege for it.

[codecov[bot]]

Codecov Report

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

Project coverage is 80.88%. Comparing base (59f50f2) to head (22bde57). Report is 118 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6843      +/-   ##
==========================================
+ Coverage   80.06%   80.88%   +0.82%     
==========================================
  Files         190      198       +8     
  Lines       37181    37103      -78     
  Branches     9450     9684     +234     
==========================================
+ Hits        29770    30012     +242     
- Misses       2997     3220     +223     
+ Partials     4414     3871     -543     

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