Implement support for the JWK format

[oleiade]

The Webcrypto API specification has support for the JWK key serialisation format. However, it is currently not implemented in this extension for a collection of reasons. If those were to be addressed, the implementation of the JWK format would therefore become unblocked.

Blocker Reasons

• JWK has no official support in the Go standard library: we see it as a security issue not having an officially supported and reviewed implementation available.
• The test suite depending on JWK, such as import/export depend on the btoa WebAPI function, which is not available in Goja. We could circumvent this issue by implementing an alternative ourselves, but it would take us further from being as close as possible from the original WebPlatform test suite.