Add test to exercise excluded headers in aws-sigv4 (#1890)

* Add test for excluded headers list

This commit adds a test for the functionality added in #1381. Technically,
there was an existing test `presigning_header_exclusion` that exercised
the said functionality but it only covered the behavior partially, i.e.
only a case where the `excluded_headers` field in `SigningSettings`
contained just `user-agent`.

The new test will randomly add headers to `excluded_headers` and verify
the functionality works as expected.

* Update CHANGELOG.next.toml

* Update CHANGELOG.next.toml

Co-authored-by: John DiSanti <jdisanti@amazon.com>

Co-authored-by: Saito <awsaito@c889f3b5ddc4.ant.amazon.com>
Co-authored-by: John DiSanti <jdisanti@amazon.com>