Add official asdf plugin support for RTK

### Is your feature request related to a problem? Please describe

RTK is a high-performance CLI proxy that reduces LLM token consumption by 60–90% by filtering and compressing command outputs before they reach AI coding assistants. It supports 100+ commands and integrates with tools like Claude Code, Gemini CLI, Cursor, Copilot, Codex, Windsurf, Cline, OpenCode, and others.

Currently, RTK provides multiple installation methods, including Homebrew, Cargo, shell installer scripts, and pre-built binaries, but there is no official/community-supported asdf plugin available.

### Describe the proposed solution

I created an asdf plugin repository for RTK: https://github.com/alanmatiasdev/asdf-rtk

```
asdf plugin add rtk https://github.com/alanmatiasdev/asdf-rtk.git
asdf install rtk latest
asdf global rtk latest
```

The plugin installs official RTK releases and integrates naturally with standard asdf workflows.

### Describe similar `asdf` features and why they are not sufficient

RTK already offers excellent installation support through:

- Homebrew
- Cargo
- install.sh
- pre-built binaries

However, none of these solutions provide native version management integration for developers already standardized on asdf.

### Describe other workarounds you've considered

Current alternatives include:

- Manual binary installation
- Homebrew installation
- Cargo installation
- Internal shell scripts
- Docker-based environments

While these approaches work well, they do not provide the same seamless version management and environment reproducibility that asdf users expect.

Review the proposed plugin repository at https://github.com/alanmatiasdev/asdf-rtk and the asdf-plugins contribution workflow first. Confirm that the plugin installs official RTK releases and that the documented `asdf plugin add`, `asdf install`, and `asdf global` commands work; the work is done when RTK is available through the community plugin directory.

Written by the indexing model from the issue text.