Support Titan project descriptor files

[5nord]

Support Titan project descriptor files.

Description

For IDE features like go-to-defintion, we need to know where the TTCN-3 files are which build the test suite.
This is what the package project is for. We only support ntt manifest files at the moment, but being able to read Titan project descriptor file as well would improve the experience for Titan users.

We should probably split this issue into smaller task. I would suggest roughly this:

• [x] create the structs to unmarshal TDP files.
• [ ] support TDP files for adapters/imports only.
• [ ] support TDP files for test suite root configuration.
• [ ] extend project.Discovery to support root TDP-files.

[5nord]

We should think about incorporating Titan Runtime Configurations Files as well.