Create Service Layer for Airavata

[yasithdev]

Currently, the core airavata logic is implemented in the thrift handlers, which makes it difficult to add new APIs, like Spring HTTP handlers, without going through the thrift handlers. This PR creates a separate service layer to capture all airavata functionalities, and the thrift handlers directly invoke them.

[yasithdev]

@lahirujayathilake thanks! I made most of the changes suggested. things missing are:

• i didn't turn the // FIXME and // TODO into unsupported exceptions, since im not sure if it would impact the rest of places that use them.