Add -InstallMode support to get-aspire-cli release installer scripts

**Summary**

`eng/scripts/get-aspire-cli-pr.{sh,ps1}` supports `-InstallMode Archive | Tool | WinGet | Homebrew` (and with #17761, `Npm`). The public release installer `eng/scripts/get-aspire-cli.{sh,ps1}` has no install-mode parameter at all — it's hardcoded to the archive download path. Users who want a non-archive install have to know about the route's native tool (`dotnet tool install -g`, `winget install`, `brew install`, `npm install -g`) and run it themselves; there's no unified release-installer command.

We should bring `--install-mode` to the release installer with the same set of modes the PR installer supports, so the documented "one-liner to install Aspire" can also be the one-liner that picks the user's preferred route.

**Background**

The release installer is the canonical install script linked from the get-started docs (`https://aspire.dev/...`, README). Today it always downloads the latest signed archive from the GitHub release and extracts it under `~/.aspire/bin/`. Users who'd prefer to install via the dotnet global tool, WinGet, Homebrew, or (with #17297) npm have to abandon the install script and run the route-native command directly. That fragments the install experience and means the README needs N install snippets instead of one.

The PR installer already does mode dispatch (`eng/scripts/get-aspire-cli-pr.ps1:152-157`), so the dispatch pattern is established. The release installer would use the same `-InstallMode` parameter with the same value set.

**Acceptance criteria**

- `get-aspire-cli.{sh,ps1} -InstallMode Archive | Tool | WinGet | Homebrew | Npm` selects the install route.
- Default remains `Archive` (no behavior change for existing users / docs).
- Each mode invokes the matching native tool command for the latest released version, with the same host-platform / `--Architecture` / `--OS` / `--Force` / `--InstallPath` parity rules the PR installer already enforces.
- The release docs and README get a section showing all the modes once #17297 ships npm and this lands.

**Out of scope**

- Changing how the release artifacts are produced or signed (already handled by the release pipeline).
- Auto-selecting a "best" install mode based on host environment (e.g., prefer `Homebrew` on macOS) — that's a separate UX question. This issue is just about making the script accept the user's choice.

**Related**

- #17761 — Add `-InstallMode Npm` to the PR installer (`get-aspire-cli-pr`). Should land first so the dispatch pattern for npm is established; this issue then ports the same code path to the release installer.