Allow direct download client to specify a shorter signed URI TTL

When you request a direct download URI from cloud blob storage, the TTL that is specified for the URI is set to a default value that is specified in configuration.

The proposal here is to consider extending the capabilities of requesting a direct download URI such that a client can specify their own TTL, so long as that TTL does not exceed a maximum value specified in the configuration.

In other words - suppose the default configured TTL is 1800 (30 minutes) and the configured max TTL is 3600 (60 minutes).  In this case:

• A client that does not specify any other TTL value would get URIs that expire in 1800 seconds.
• A client that chooses to specify a TTL value could provide any value greater than 0 but less than or equal to 3600 seconds.
• Specifying a TTL value of over 3600 would be an error condition.
• Specifying a TTL value of 0 would be an error condition.
• Specifying a TTL value of less than 0 would be the same as the default value (-1), meaning to use the configured value (i.e. 1800 seconds).

The maximum TTL value would be considered an optional configuration value.  If not specified, the default configured TTL would be used (which is required to enable the direct download feature).  If specified but lower than the default, the actual max would be the default value (i.e. max(default TTL, max TTL)).

What should happen if an error condition should occur?  Typically in the direct download code an error results in the return of a null value for the URI, accompanied by a log message - so that'd be my proposal for this situation also.