Allow changing the public exponent for RSA keys

[ionut-arm]

In the Transient Object Context we do not allow the clients to set a custom public exponent, always defaulting to 0 (i.e. 65537).
While the specs mention that this parameter is optional for TPM implementations, this crate should ideally support customisation for any clients that want to use it.

[hug-dev]

The RsaExponent could probably be helpful there.

[puiterwijk]

I'm not sure that we should support this. From a crypto point of view, I think it's easy for people to Get Crypto Wrong(R) by picking the wrong exponent.
Though from a low-level I guess we should support it.

[Superhepper]

I think we should support it but make it cumbersome. To avoid some one doing it by mistake.

[hug-dev]

Ok, closing for now as not something we will likely do anytime soon