impl(oauth2): create AWS V4 signed request

[coryan]

Create the AWS V4 signed request as a JSON object because it is easier to test. In the actual implementation this becomes a string, that is then URL-encoded (yikes!) and passed to Google's Security Token Service.

Part of the work for #5915

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This change is 

[google-cloud-cpp-bot]

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[codecov]

Codecov Report

Base: 93.91% // Head: 93.91% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (b17e77c) compared to base (03cd5fa).
Patch coverage: 98.87% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #10452   +/-   ##
=======================================
  Coverage   93.91%   93.91%           
=======================================
  Files        1614     1614           
  Lines      146618   146707   +89     
=======================================
+ Hits       137700   137786   +86     
- Misses       8918     8921    +3     

Impacted Files	Coverage Δ
...internal/external_account_token_source_aws_test.cc	99.51% <98.11%> (-0.21%)	⬇️
...loud/internal/external_account_token_source_aws.cc	94.90% <100.00%> (+1.51%)	⬆️
...bigtable/examples/bigtable_hello_instance_admin.cc	81.00% <0.00%> (-2.00%)	⬇️
google/cloud/pubsub/subscriber_connection_test.cc	98.78% <0.00%> (-1.22%)	⬇️
...le/cloud/storage/internal/curl_download_request.cc	88.29% <0.00%> (-1.01%)	⬇️
...sub/internal/batching_publisher_connection_test.cc	99.44% <0.00%> (-0.19%)	⬇️
...cloud/pubsub/internal/subscription_session_test.cc	97.99% <0.00%> (-0.17%)	⬇️
google/cloud/pubsub/samples/samples.cc	87.93% <0.00%> (+0.07%)	⬆️
google/cloud/storage/parallel_upload.cc	98.62% <0.00%> (+0.34%)	⬆️
google/cloud/bigtable/async_read_stream_test.cc	97.99% <0.00%> (+0.66%)	⬆️
... and 1 more

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[scotthart]

Reviewed all commit messages.
Reviewable status: 0 of 3 files reviewed, 1 unresolved discussion (waiting on @coryan)

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google/cloud/internal/external_account_token_source_aws.h line 113 at r1 (raw file):

    ExternalAccountTokenSourceAwsSecrets const& secrets,
    std::chrono::system_clock::time_point now, std::string const& target,
    bool debug);

Maybe default this to false?

Code quote:

bool debug

[google-cloud-cpp-bot]

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