Support for Azure Connection Strings and Shared Access Signature (SAS) auth

[shaunco]

The azure/auth package doesn't seem to have support for shared access keys or connection strings. Most of the Azure data plane APIs make use of both.

[shaunco]

Sorry, I overlooked SAS in the main autorest package. Connection String support would still be nice, but easy enough to parse on my own.

[shaunco]

Looks like I closed this too soon. SASTokenAuthorizer requires that the SAS token be generated in advance by the caller, which leaves both SAS token generation (and regeneration after expiration) a bit messy. Would be really nice to be able to pass the keyname and key and have SASTokenAuthorizer generate it for each request.

Happy to submit my revised version as a PR.