Task: Implement the tufstorage in `internal`

[kairoaraujo]

Implement the internal/tufstorage backend in Archivista
This backend will manage

• Add an Attestation or Policy to the TUF (RSTUF) metadata
• Manage the TUF task status

Note 1: RSTUF uses asynchronous tasks to handle the requests, but Archivista uses synchronous operations
The suggestion is to store the Attestation/Policy in the Object Storage -> TUF -> SQL
This will give consistency to the Attestation/Policy as SQL is the interaction to query the content, and availability in the other storages (Object and TUF) is required.

Note 2: It doesn't enable yet the TUF usage.

Parent feature

• Feature|Experimental: Add support to add Policies to TUF #159