signed-web-bundles: Use stronger key types in tests

Instead of using `std::vector<uint8_t>` to represent keys in tests, use
`Ed25519PublicKey` for public keys and `std::array<uint8_t, 64>` for
private keys (there is no separate wrapper type for private keys given
that private keys are only used in tests).

Bug: None
Change-Id: Icf297fa5a307cfe72f3185c65897254e9e7d016f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4020302


Reviewed-by: Phillis Tang <phillis@chromium.org>
Commit-Queue: Christian Flach <cmfcmf@chromium.org>
Reviewed-by: Hayato Ito <hayato@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1072166}