Implement "getPublicKey" operations of ECDSA and ECDH. The steps are
implemented at the new submodule `ec_common` shared by ECDSA and ECDH.
To make the operations functioning, they are also registered at
`GetPublicKeyAlgorithm`.
Specification:
https://wicg.github.io/webcrypto-modern-algos/#SubtleCrypto-method-getPublicKey
Testing: Pass WPT tests that were expected to fail.
Fixes: Part of #43072
Signed-off-by: Kingsley Yung <kingsley@kkoyung.dev>