Add support for native image

### Description

Add graalvm reachability metadata to the project.

### Motivation

Due to the heavy use of reflection, it is currently very hard to use sshd in native images. With reachability metadata, any application that consumes this library would automatically include the necessary classes and resources to create a native image.

### Alternatives considered

Alternatively, metadata can be contributed to the [reachability metadata repository](https://github.com/oracle/graalvm-reachability-metadata).

### Additional context

_No response_

No project files, tests, or entry points are named. Start by reviewing the project's reflection-heavy areas and the GraalVM reachability metadata format, then verify that a consuming application can create a native image with the required classes and resources included.

Written by the indexing model from the issue text.