Aspire CLI: allow overriding terminal attach leader key (Ctrl+B)

## Feature request
`aspire terminal attach` currently uses **Ctrl+B** as the leader key for CLI-level controls. This conflicts with TUIs that already bind Ctrl+B, making those interactions hard or impossible to test through Aspire terminal attach.

## Problem
When the attached app/TUI expects Ctrl+B, the Aspire attach layer intercepts it as the leader key.

## Proposal
Add a CLI option to override the leader key, for example:

```bash
aspire terminal attach --leader-key CTRL-L
```

## Goals
- Allow users to choose a non-conflicting leader key per attach session.
- Keep a default leader key for current behavior.
- Clearly document accepted key formats and invalid combinations.

## Why this matters
This unblocks testing and interactive use of TUIs that rely on Ctrl+B bindings while still preserving Aspire attach controls.