Key Management API

[kkloberdanz]

CXX-2425

• Add support for new custom key material in DataKeyOpts.
• Add support for new Key Management API functions in ClientEncryption.

[kevinAlbs]

Commits in DRIVERS-2017 also include tests to validate the unified test runner implementation.
I suggest copying the following test files to data/unified-valid-pass from the last modified commit (mongodb/specifications@d8be9ce) which includes the test simplification of CXX-2538:

source/unified-test-format/tests/valid-pass/kmsProviders-explicit_kms_credentials.json
source/unified-test-format/tests/valid-pass/kmsProviders-placeholder_kms_credentials.json
source/unified-test-format/tests/valid-pass/kmsProviders-unconfigured_kms.json
source/unified-test-format/tests/valid-pass/kmsProviders-mixed_kms_credential_fields.json

[eramongodb]

Please replace the const std::string& parameters for get_key_by_alt_name and remove_key_alt_name with bsoncxx::string::view_or_value and revert the return type of these functions + the get_key and add_key_alt_name functions to stdx::optional<bsoncxx::document::value>. Consistency with public API was specific to the string-like parameters, not the return type. stdx::optional<bsoncxx::document::value> was the correct return type for these functions.

In addition to some minor suggestions to avoid redundant copies, otherwise LGTM.