Add new service call to provide signed access to logical files

Currently meta data is retrieved by DFUInfo, which is used by EclWatch/IDE to display file details.

The Esp DFUInfo request method also does other things like updating file descriptions and finding files based on query names etc.

We should introduce a new Esp method that is specifically for requesting meta data intended to be used by clients to read data.
The response will be similar to the normal DFUInfo response for now, but is likely to diverge and may have additional semantics in the future (e.g. locking).

The response will include various tags from the request
+ expiry (possibly restricted by server to not be excessively large)
+ access type
And info that the service has retrieved and/or generated
+ user
+ file part meta info
+ record type mata info (may be included in file part meta info for now)
+ a signature based on all the above.

The request should look something like:

DFUReadAccessRequest
{
    string Name;
    string Cluster;
    int Expiry; // in seconds
    string AccessType; // for now this is probably only "read"
    string JobId; // job id from client
    bool refresh(false); // only return signature
};

The response is going need to look something like this:

<Response @xmlns:dfs="@xmlns:dfs">
 <dfs:SignedContent>
  <Filename>abc</Filename>
  <User>jake</User>
  <Expiry>10000</Expiry>
  <AccessType>Read</AccessType>
  <JobId>myjob1</JobId>
  <FileMetaInfo>
....
  </FileMetaInfo>
 </dfs:SignedContent>
 <dfs:Signature>dGhpcyBpcyBub3QgYSBzaWduYXR1cmUK<dfs:Signature>
</Response>