feat: add in memory implementation of HeightIndex Database

[DracoLi]

Why this should be merged

Adds an in memory implementation of database.HeightIndex. The main use of this will be in tests where we don't need a on-disk height-indexed database.

How this works

The package is in database/heightindexdb/memdb. It uses a map to store data by height.

How this was tested

Unit tests

Need to be documented in RELEASES.md?

No

[Copilot]

Pull Request Overview

This PR introduces an in-memory implementation of the BlockDatabase interface for testing purposes, allowing tests to avoid using on-disk storage when not necessary.

• Adds a new MemoryDatabase struct that implements the BlockDatabase interface
• Provides thread-safe operations using read-write mutexes
• Includes proper error handling for closed database states and empty blocks

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[joshua-kim]

I'm also confused here, I thought Get returned nil if you inserted empty - why does Has return nil or empty?

[DracoLi]

this is to clarify Has returns true if the value at the key is nil or empty.

[StephenButtolph]

comment nits then lgtm