CertificateAssetDownloader should be updated to use HttpClient.Send

[stephentoub]

CertificateAssetDownloader on Unix currently uses HttpClient via reflection to download data:

runtime/src/libraries/System.Security.Cryptography.X509Certificates/src/Internal/Cryptography/Pal.Unix/CertificateAssetDownloader.cs

Line 126 in 2981ad9

private static Func<string, Task<byte[]>>? CreateDownloadBytesFunc()

It does this via the asynchronous GetByteArrayAsync method, blocking waiting for it to complete. This should instead be updated to use the newly-added synchronous Send method so as to avoid blocking a thread pool thread for the sync-over-async operation. (This will be a bit more involved, but it's doable.)

cc: @bartonjs, @ManickaP

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