Add ttl as public field for Document

[ptpaterson]

closes #240

Problem

ttl can be set on a Document explicitly at creation time, updated later, or be set automatically at creation if a Collection's ttl_days is set. We should add this common field to the Document class.

Solution

Add ttl as a public, optional field to the Document class.

Result

The ttl field can now be accessed in Typescript projects without adding it to user types.

Out of scope

N/A

Testing

Added unit and integration test.

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