Imported RSA keys should have modulusLength and publicExponent

[tniessen]

Generated RSA keys correctly set key.algorithm to an object with modulusLength and publicExponent properties. However, imported RSA keys are missing these properties since there is no easy way to determine them based on a KeyObject.

This could also be solved via nodejs/node#26854.

[tniessen]

Possible solution in nodejs/node#30045.