[FR]: Add an `integrity` checksum argument to aspect launcher methods

### What is the current behavior?

https://github.com/aspect-build/aspect-cli/blob/362d646d3ce1a6bd0e1a26d8d7c817d10f7f4919/crates/aspect-launcher/src/main.rs#L155

The current behavior (marked `FIXME`) does not accept any integrity checksums for any of the three supported launcher methods (http, github, local).

### Describe the feature

As is true in the rest of the Bazel ecosystem, it is useful for a variety of reasons to lockdown tools and dependencies used in a repo. The aspect-cli does not perform any checksum verification on the retrieved binaries, leaving room for a malicious actor to potentially substitute alternative binaries with undesirable behavior.

I'm imagining it would look something like:

```python
version("2026.27.7", integrity = "sha256-yUWuFQOLEbOFxboIBRNdoDp8pF7/+mivaaK9QVLxrXA=")
```

But I don't think the exact API shape matters as much as having confidence that the binary that's about to be executed matches the one we thought we were downloading.

Start at crates/aspect-launcher/src/main.rs around line 155, then trace the three launcher methods named in the issue: http, github, and local. Review how the version("2026.27.7", integrity = ...) API could reach each method, and verify completion by checking that retrieved binaries are rejected when their supplied checksum does not match.

Written by the indexing model from the issue text.