[BEAM-9547] Thorough testing for global aggregations

[TheNeuralBit]

In the same spirit as #14850 I added some thorough parameterized tests for global aggregations, separated into their own test suite AggregationTests. Not surprisingly this knocked loose a few unhandled edge cases, also fixed in this PR:

• We don't need a specialized implementation of nunique in DataFrame, it can be done with _agg_method. We can add a distributed implementation in Series though, based on drop_duplicates, and it will be used through DataFrame.agg.
• Added logic to direct DataFrame.agg to specialized implementations for corr, cov, quantile and size.
• Direct Series.agg to specialized implementations for corr, cov.

ValidatesRunner compliance status (on master branch)

Lang	ULR	Dataflow	Flink	Samza	Spark	Twister2
Go	---			---		---
Java
Python	---			---		---
XLang				---		---

Examples testing status on various runners

Lang	ULR	Dataflow	Flink	Samza	Spark	Twister2
Go	---	---	---	---	---	---	---
Java	---		---	---	---	---	---
Python	---	---	---	---	---	---	---
XLang	---	---	---	---	---	---	---

Post-Commit SDK/Transform Integration Tests Status (on master branch)

Go	Java	Python

Pre-Commit Tests Status (on master branch)

---	Java	Python	Go	Website	Whitespace	Typescript
Non-portable
Portable	---			---	---	---

See .test-infra/jenkins/README for trigger phrase, status and link of all Jenkins jobs.

GitHub Actions Tests Status (on master branch)

See CI.md for more information about GitHub Actions CI.

[TheNeuralBit]

R: @rohdesamuel

[codecov]

Codecov Report

Merging #14893 (e41b1f4) into master (f207ad1) will increase coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #14893      +/-   ##
==========================================
+ Coverage   83.76%   83.79%   +0.03%     
==========================================
  Files         435      435              
  Lines       58365    58434      +69     
==========================================
+ Hits        48887    48966      +79     
+ Misses       9478     9468      -10     

Impacted Files	Coverage Δ
...e_beam/portability/api/beam_interactive_api_pb2.py
..._beam/testing/benchmarks/nexmark/queries/query8.py
..._beam/testing/benchmarks/nexmark/queries/query4.py
...runners/interactive/display/pcoll_visualization.py
..._beam/testing/benchmarks/nexmark/queries/query3.py
...apache_beam/typehints/native_type_compatibility.py
...am/testing/benchmarks/chicago_taxi/trainer/taxi.py
...ld/srcs/sdks/python/apache_beam/utils/processes.py
...n/apache_beam/examples/cookbook/bigquery_schema.py
...ks/python/apache_beam/runners/worker/statecache.py
... and 860 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update f207ad1...e41b1f4. Read the comment docs.