ESQL: AVG aggregation tests and ignore complex surrogates (#110579)

Some work around aggregation tests, with AVG as an example:
- Added tests and autogenerated docs for AVG
- As AVG uses "complex" surrogates (A combination of functions), we can't trivially execute them without a complete plan. As I'm not sure it's worth it for most aggregations, I'm skipping those cases for now, as to avoid blocking other aggs tests.

The bad side effect of skipping those tests is that most tests in AvgTests are actually ignored (74 of 100)

Author: ivancea

docs/reference/esql/functions/aggregation-functions.asciidoc

@@ -8,7 +8,7 @@
 The <<esql-stats-by>> command supports these aggregate functions:
 
 // tag::agg_list[]
-* <<esql-agg-avg>>
+* <<esql-avg>>
 * <<esql-agg-count>>
 * <<esql-agg-count-distinct>>
 * <<esql-agg-max>>
@@ -23,7 +23,6 @@ The <<esql-stats-by>> command supports these aggregate functions:
 * experimental:[] <<esql-agg-weighted-avg>>
 // end::agg_list[]
 
-include::avg.asciidoc[]
 include::count.asciidoc[]
 include::count-distinct.asciidoc[]
 include::max.asciidoc[]
@@ -33,6 +32,7 @@ include::min.asciidoc[]
 include::percentile.asciidoc[]
 include::st_centroid_agg.asciidoc[]
 include::sum.asciidoc[]
+include::layout/avg.asciidoc[]
 include::layout/top.asciidoc[]
 include::values.asciidoc[]
 include::weighted-avg.asciidoc[]