Add get_app_at_slot method to retrieve descriptor for any index

[Ctru14]

For the BootableRegionDescriptors object to return the AppImageDescriptor from an app at any specified index. The new get_app_at_slot(idx) method will verify the index is within the number of app slots then return the specified app descriptor.

[felipebalbi]

Please rebase on current main and make sure to combine commits such that each commit in your PR compiles and runs without known issues.

[Copilot]

Pull Request Overview

This pull request adds a new method to the BootableRegionDescriptors struct to allow retrieval of a descriptor for a specified app slot while ensuring the provided index is valid. It also updates the Rust minimal supported version (msrv) in the workflow configuration.

• Added the get_app_at_slot method in src/lib.rs to retrieve an app descriptor at a given index.
• Updated the msrv from 1.79 to 1.84 in the GitHub Actions workflow.

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/lib.rs	Added the get_app_at_slot method with an index bounds check and call to from_region.
.github/workflows/check.yml	Updated the msrv version to support new namespaced features.

Comments suppressed due to low confidence (1)

src/lib.rs:192

• [nitpick] Consider renaming the parameter 'app_index' to 'app_slot' for consistency with other code references such as the 'app_slot' parameter in from_region and 'num_app_slots' in the header.

pub fn get_app_at_slot(&self, app_index: u32) -> Result<AppImageDescriptor, ParseError> {

[felipebalbi]

Not all commits compile. Additionally, you still have a few clippy errors.