MultibaseHelper

Returns something like did:key:mEpA... with the x and y values appended in Base64. This translates to Base64(0x12, 0x90, EC-P-256-Key). Note that 0x1290 is not an official Multicodec prefix, but there seems to be none for uncompressed P-256 key. We can't use the compressed format, because decoding that would require some EC Point math...