Add plan-time chunk exclusion for real-time CAggs

[jnidzwetzki]

The watermark function for CAggs is declared as STABLE since the value
of the function changes after every CAgg refresh. The function
volatility prevents the planner from replacing the function invocation
with a constant value and executing plan time chunk exclusion. This
leads to high planning times on hypertables with many chunks.

This PR replaces the function invocation with a constant value to allow
plan time exclusion of chunks. We perform the replacement at plan time
instead of changing the function volatility to IMMUTABLE, because we
want to control the constification. Only queries that access the
underlying hypertable in a query (i.e., no queries like SELECT cagg_watermark(...)
without any FROM condition) are rewritten. This is
done to make sure that the query is properly invalidated when the
underlying table changes (e.g., the watermark is updated) and the query
is replanned on the subsequent execution.

Fixes: #6105, #6194, #6321

---

Benchmarks

https://grafana.ops.savannah-dev.timescale.com/d/NdmLnOk4z/compare-benchmark-runs?orgId=1&var-branch=main&var-run1=3158&var-run2=3160&var-threshold=0.02

[codecov]

Codecov Report

Attention: 14 lines in your changes are missing coverage. Please review.

Comparison is base (d878b9e) 79.72% compared to head (14a6b70) 79.82%.
Report is 1 commits behind head on main.

❗ Current head 14a6b70 differs from pull request most recent head ddd3da6. Consider uploading reports for the commit ddd3da6 to get more accurate results

Files	Patch %	Lines
tsl/src/continuous_aggs/planner.c	90.07%	0 Missing and 13 partials ⚠️
tsl/src/planner.c	80.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6325      +/-   ##
==========================================
+ Coverage   79.72%   79.82%   +0.09%     
==========================================
  Files         188      189       +1     
  Lines       36739    36841     +102     
  Branches     9287     9320      +33     
==========================================
+ Hits        29291    29408     +117     
+ Misses       3114     3101      -13     
+ Partials     4334     4332       -2     

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[svenklemm]

LGTM

[timescale-automation]

Automated backport to 2.13.x not done: cherry-pick failed.

Git status

HEAD detached at origin/2.13.x
You are currently cherry-picking commit b63fbad33.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	new file:   .unreleased/enhancement_6325
	modified:   src/guc.h
	modified:   tsl/src/continuous_aggs/CMakeLists.txt
	new file:   tsl/src/continuous_aggs/planner.c
	new file:   tsl/src/continuous_aggs/planner.h
	modified:   tsl/src/init.c
	modified:   tsl/src/planner.c
	modified:   tsl/test/expected/cagg_permissions.out
	deleted:    tsl/test/expected/cagg_query-14.out
	deleted:    tsl/test/expected/cagg_query-15.out
	deleted:    tsl/test/expected/cagg_query-16.out
	renamed:    tsl/test/expected/cagg_query-13.out -> tsl/test/expected/cagg_query.out
	modified:   tsl/test/expected/cagg_union_view-13.out
	modified:   tsl/test/expected/cagg_union_view-14.out
	modified:   tsl/test/expected/cagg_union_view-15.out
	modified:   tsl/test/expected/cagg_union_view-16.out
	new file:   tsl/test/expected/cagg_watermark-13.out
	new file:   tsl/test/expected/cagg_watermark-14.out
	new file:   tsl/test/expected/cagg_watermark-15.out
	new file:   tsl/test/expected/cagg_watermark-16.out
	deleted:    tsl/test/expected/cagg_watermark.out
	modified:   tsl/test/expected/jit-13.out
	modified:   tsl/test/expected/jit-14.out
	modified:   tsl/test/expected/jit-15.out
	modified:   tsl/test/expected/jit-16.out
	new file:   tsl/test/isolation/expected/cagg_watermark_concurrent_update.out
	new file:   tsl/test/isolation/expected/cagg_watermark_concurrent_update_1.out
	modified:   tsl/test/isolation/specs/CMakeLists.txt
	new file:   tsl/test/isolation/specs/cagg_watermark_concurrent_update.spec
	modified:   tsl/test/sql/cagg_permissions.sql
	renamed:    tsl/test/sql/cagg_query.sql.in -> tsl/test/sql/cagg_query.sql
	deleted:    tsl/test/sql/cagg_watermark.sql
	new file:   tsl/test/sql/cagg_watermark.sql.in

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   src/cross_module_fn.c
	both modified:   src/cross_module_fn.h
	both modified:   src/guc.c
	both modified:   src/planner/planner.c
	both modified:   src/ts_catalog/continuous_aggs_watermark.c
	both modified:   src/ts_catalog/continuous_aggs_watermark.h
	both modified:   tsl/src/continuous_aggs/common.c
	both modified:   tsl/src/continuous_aggs/common.h
	both modified:   tsl/src/planner.h
	both modified:   tsl/test/expected/continuous_aggs-13.out
	both modified:   tsl/test/expected/continuous_aggs-14.out
	both modified:   tsl/test/expected/continuous_aggs-15.out
	both modified:   tsl/test/expected/continuous_aggs-16.out
	both modified:   tsl/test/sql/CMakeLists.txt

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