feat(common): Add converting constructors for future

[dpcollins-google]

When the value type of a future is convertible to the value type of another, this allows the futures to interconvert.

This is useful with, for example, make_ready_future, where one could return make_ready_future(absl::nullopt) for a return type of future<absl::optional<Result>>

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This change is 

[google-cloud-cpp-bot]

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[codecov]

Codecov Report

Merging #8329 (fd03071) into main (541eb09) will decrease coverage by 0.01%.
The diff coverage is 91.66%.

@@            Coverage Diff             @@
##             main    #8329      +/-   ##
==========================================
- Coverage   95.09%   95.08%   -0.02%     
==========================================
  Files        1360     1360              
  Lines      121124   121136      +12     
==========================================
- Hits       115185   115181       -4     
- Misses       5939     5955      +16     

Impacted Files	Coverage Δ
google/cloud/future_generic_test.cc	98.58% <80.00%> (-0.23%)	⬇️
google/cloud/future_generic.h	100.00% <100.00%> (ø)
google/cloud/future_void.h	100.00% <100.00%> (ø)
google/cloud/future_void_test.cc	98.18% <100.00%> (+0.01%)	⬆️
...e/cloud/spanner/testing/cleanup_stale_instances.cc	61.29% <0.00%> (-19.36%)	⬇️
google/cloud/bigtable/internal/common_client.h	94.02% <0.00%> (-5.98%)	⬇️
google/cloud/internal/filesystem.h	91.83% <0.00%> (-4.09%)	⬇️
google/cloud/internal/filesystem.cc	92.85% <0.00%> (-1.43%)	⬇️
google/cloud/pubsub/subscriber_connection_test.cc	97.22% <0.00%> (-0.70%)	⬇️
google/cloud/bigtable/async_read_stream_test.cc	97.32% <0.00%> (-0.67%)	⬇️
... and 4 more

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[coryan]

The robots have things to say, I agree with my robot overlords.

[google-cloud-cpp-bot]

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[coryan]

Can you please rebase? We have been making a lot of changes to our builds and you are missing some support scripts / changes.

[coryan]

Ping.

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[dpcollins-google]

Can you please rebase? We have been making a lot of changes to our builds and you are missing some support scripts / changes.

Done.