Use Name reference for secure files instead of ID

[aklakina]

Current mechanism

Currently the tasks use secure files objects referenced by id.

Proposed mechanism

The tasks should reference every secure file by name and only resolve the id when it is necessary like creating a download ticket.

Justification

The name of the secure file is also a unique identifier. In a project only 1 secure file can exist with any given name.
The mechanism of referencing by id also breaks every task that was storing the id as a ref when someone wants to update the content of the secure file because the update itself removes the secure file and then recreates it with the same name.

Fix

#307

### Tasks
- [ ] https://github.com/microsoft/azure-pipelines-tasks-common-packages/pull/307