Remove JobInfo hierarchy, add JobConfiguration hierarchy

[mziccard]

This PR removes JobInfo hierarchy and replaces it with a hierarchy on JobConfiguration. This change is needed to make functional Job extend JobInfo. Notes:

• A configuration object (CopyJobConfiguration, ExtractJobConfiguration, LoadJobConfiguration or QueryJobConfiguration) must be provided to create a JobInfo object.
• LoadJobConfiguration also extends LoadConfiguration which is used for resumable uploads
• JobConfiguration is an interface with only a type() method since configurations have no common data (this is also required to make LoadJobConfiguration extend LoadConfiguration).

[aozarov]

Done withe the review. looks good. Main comment (as discussed verbally) is the suggestion to make JobConfiguration interface an abstract class (and use package scope methods to set or get from pb).

[aozarov]

Looks great! Do you plan to do the same for BaseTableInfo ?

BTW, looks like BigQueryException has 2 unused import statements.