tapdb: refactor logic to reduce duplication between IgnoreTree and BurnTree

Previously, the `BurnUniverseTree` and `IgnoreUniverseTree` implementations
contained significant duplicated logic for common operations such as:
- Deriving a universe identifier from an asset specifier.
- Calculating the total sum of leaf values in a tree.
- Querying specific leaves and generating their MS-SMT inclusion proofs.
- Listing all leaves within a specific universe namespace.

This commit introduces several generic helper functions within
`tapdb/universe.go` to centralize this logic:
- `specifierToIdentifier`: Creates a universe identifier for a given
  specifier and proof type.
- `getUniverseTreeSum`: Calculates the sum of a universe tree root.
- `queryUniverseLeavesAndProofs`: A generic function to query leaves,
  decode them, fetch MS-SMT proofs, and build authenticated results.
- `listUniverseLeaves`: A generic function to list and decode all leaves
  in a namespace.

The `BurnUniverseTree` and `IgnoreUniverseTree` methods (`Sum`,
`QueryBurns`, `ListBurns`, `QueryTuples`, `ListTuples`) have been
refactored to utilize these new helpers. This significantly reduces
code duplication and improves the maintainability of universe tree
interactions.

The `IgnoreTree` interface and related tests were updated to reflect
changes in the `ListTuples` return type.

Author: Roasbeef