Add parameter to specify SAS expiration as timespan

**Is your feature request related to a problem? Please describe.**
The various commands to generate Shared Access Signatures, such as `az storage share generate-sas` only support specifying the signature expiration time using the `--expiry` argument, which takes an absolute date/time value in a specific format.

This is inconvenient many times when you're trying to quickly generate a SAS.

**Describe the solution you'd like**
It would be great to be able to specify expiration time as a timespan value, rather than an absolute time.

**Describe alternatives you've considered**
This could be done by either making the `--expiry` argument support timespan values, or by adding a separate `--expires-in` argument or similar.

For example:
`az storage share generate-sas ..... --expiry PT1H`

(or whatever other easy to parse timespan format is suitable here).