Standardized way to represent and query expiry for any or token, or other asset

[bhess]

Crypto-asset types "certificate" and "key" already have properties for expiry. This issue is to add the same functionality to "relatedCryptoMaterial", which relates to other assets (e.g. tokens, password).

Thanks to @lfrancke for the suggestion.

[bhess]

I was wondering why we wouldn't just merge the asset-types "key" and "relatedCryptoMaterial". This would address this issue and add some more flexibility:

The properties currently specific to "key" can also be relevant for "relatedCryptoMaterial":
"state"
"keyAlgorithmRef"
"securedBy"
"creationDate"
"activationDate"
"updateDate"
"expirationDate"

Vice versa, the properties currently specific to "relatedCryptoMaterial" can also be relevant for "key":
"format"
"securedBy"
"value"

Any thoughts @n1ckl0sk0rtge ?

[bhess]

Suggested change is in PR #35