Add VuMark database type and template target support

[adamtheturtle]

Background

In the real Vuforia, the database type (Cloud Reco vs VuMark) is set when creating the database in the Target Manager. A VuMark database only contains VuMark template targets; a Cloud Reco database only contains image targets.

Proposed changes

1. VuforiaDatabase gains a database type field (Cloud Reco vs VuMark), surfaced through add_database. The VuMark generation endpoint should raise InvalidTargetType when the target does not belong to a VuMark database.

2. VuMark template targets can be pre-populated by constructing a Target with a VuMark template type and passing it in the targets set when constructing VuforiaDatabase. This follows the existing pattern of allowing programmatic setup of state that is not reachable via the HTTP API (e.g. adding targets via the constructor rather than through add_target). Adding VuMark templates via the HTTP API is not supported by Vuforia — they are uploaded through the Target Manager UI as SVG files.