Use function cache to check for valid bucketing functions

[svenklemm]

Refactor the continuous aggregate validation to use our function cache
to check for bucketing function. This simplifies the code and allows
adding support for other bucketing functions like date_trunc later on.

[codecov]

Codecov Report

Merging #1549 into master will increase coverage by 0.31%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1549      +/-   ##
==========================================
+ Coverage   91.48%   91.79%   +0.31%     
==========================================
  Files         144      142       -2     
  Lines       21450    20495     -955     
==========================================
- Hits        19624    18814     -810     
+ Misses       1826     1681     -145

Flag	Coverage Δ
#cron	?
#pr	91.79% <100%> (-0.02%)	⬇️

Impacted Files	Coverage Δ
tsl/src/continuous_aggs/create.c	97.5% <100%> (+0.02%)	⬆️
src/plan_add_hashagg.c	43.33% <0%> (-45.01%)	⬇️
src/telemetry/uuid.c	84.61% <0%> (-15.39%)	⬇️
src/planner_import.c	58.65% <0%> (-11.48%)	⬇️
src/cache_invalidate.c	78.12% <0%> (-2.44%)	⬇️
src/loader/bgw_launcher.c	89.36% <0%> (-2.25%)	⬇️
src/bgw_policy/chunk_stats.c	83.92% <0%> (-1.56%)	⬇️
src/loader/loader.c	91.57% <0%> (-1.25%)	⬇️
src/histogram.c	89.02% <0%> (-1.22%)	⬇️
src/estimate.c	86.17% <0%> (-0.83%)	⬇️
... and 77 more

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