fix(common): returnable argument to .then()

[coryan]

The main motivation for these changes is to support:

future<T> F(future<T> f) {
  return f.then([](auto g) { /* stuff */; return g; });
}

This is common enough, and was broken for fairly obscure reasons. I completely changed the implementation of .then(). It was too hacky to do something else, and I think the new implementation is simpler too.

Fixes #13314 and fixes #13258

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This change is 

[codecov]

Codecov Report

Attention: 21 lines in your changes are missing coverage. Please review.

Comparison is base (66db303) 93.05% compared to head (eb4fa2e) 93.06%.
Report is 4 commits behind head on main.

Files	Patch %	Lines
google/cloud/internal/future_then_impl_test.cc	95.02%	10 Missing ⚠️
google/cloud/internal/future_then_impl.h	93.61%	6 Missing ⚠️
google/cloud/internal/future_impl.h	82.75%	5 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##             main   #13316    +/-   ##
========================================
  Coverage   93.05%   93.06%            
========================================
  Files        2140     2142     +2     
  Lines      186610   186838   +228     
========================================
+ Hits       173657   173873   +216     
- Misses      12953    12965    +12     

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[coryan]

The check-api results are false positives.

1. Evidently we no longer create an instantiation of future<future<future<void>>>::get(), we do not need to, but if anybody creates an instance it will work.

Step #3: Removed Symbols  1 
Step #3: 
Step #3: ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
Step #3: future_generic.h
Step #3: namespace google::cloud::v2_20
Step #3: future<future<future<void> > >::get ( )
Step #3: _ZN6google5cloud5v2_206futureINS2_INS2_IvEEEEE3getEv

2. This is an absl::typedef that went away:

Step #3: Problems with Data Types, Low Severity  1 
Step #3: 
Step #3: ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
Step #3: type_traits.h
Step #3: namespace absl
Step #3: [+] typedef remove_reference_t  1 

[coryan]

The warnings on Debian:bookworm (GCC 12 FWIW) look like false positives:

/usr/include/c++/12/bits/exception_ptr.h:117:33: error: '*(std::__exception_ptr::exception_ptr*)((char*)&<unnamed> + offsetof(absl::debian3::variant<absl::debian3::monostate, std::__exception_ptr::exception_ptr, google::cloud::v2_20::internal::FutureVoid, google::cloud::v2_20::internal::FutureValueRetrieved>,absl::debian3::variant<absl::debian3::monostate, std::__exception_ptr::exception_ptr, google::cloud::v2_20::internal::FutureVoid, google::cloud::v2_20::internal::FutureValueRetrieved>::<unnamed>.absl::debian3::variant_internal::VariantCopyAssignBaseNontrivial<absl::debian3::monostate, std::__exception_ptr::exception_ptr, google::cloud::v2_20::internal::FutureVoid, google::cloud::v2_20::internal::FutureValueRetrieved>::<unnamed>.absl::debian3::variant_internal::VariantMoveAssignBaseNontrivial<absl::debian3::monostate, std::__exception_ptr::exception_ptr, google::cloud::v2_20::internal::FutureVoid, google::cloud::v2_20::internal::FutureValueRetrieved>::<unnamed>.absl::debian3::variant_internal::VariantCopyBaseNontrivial<absl::debian3::monostate, std::__exception_ptr::exception_ptr, google::cloud::v2_20::internal::FutureVoid, google::cloud::v2_20::internal::FutureValueRetrieved>::<unnamed>.absl::debian3::variant_internal::VariantMoveBaseNontrivial<absl::debian3::monostate, std::__exception_ptr::exception_ptr, google::cloud::v2_20::internal::FutureVoid, google::cloud::v2_20::internal::FutureValueRetrieved>::<unnamed>.absl::debian3::variant_internal::VariantStateBaseDestructorNontrivial<absl::debian3::monostate, std::__exception_ptr::exception_ptr, google::cloud::v2_20::internal::FutureVoid, google::cloud::v2_20::internal::FutureValueRetrieved>::<unnamed>.absl::debian3::variant_internal::VariantStateBase<absl::debian3::monostate, std::__exception_ptr::exception_ptr, google::cloud::v2_20::internal::FutureVoid, google::cloud::v2_20::internal::FutureValueRetrieved>::state_)).std::__exception_ptr::exception_ptr::_M_exception_object' may be used uninitialized [-Werror=maybe-uninitialized]
  117 |       : _M_exception_object(__o._M_exception_object)
      |                             ~~~~^~~~~~~~~~~~~~~~~~~
/workspace/google/cloud/internal/future_impl.h: In member function 'void google::cloud::v2_20::internal::AsyncConnectionReadyFuture::RunIteration()':
/workspace/google/cloud/internal/future_impl.h:176:65: note: '<anonymous>' declared here
  176 |   future_shared_state() : future_shared_state([] {}, ValueType{}) {}
      |                                                                 ^
cc1plus: all warnings being treated as errors

First, it is a maybe-unitialized. Second, it is in absl::variant<> (or std::exception_ptr) we have no control over either and are supposed to initialize properly. Third, we use the version of Abseil included in this distro, which is fairly old.

I am going to suppress the warning using #pragma.