fix: update multistore to properly recognize directory markers (#125)

## What I'm changing

Bump `multistore` version to
[v0.3.1](https://github.com/developmentseed/multistore/releases/tag/v0.3.1)
to support directory markers
(developmentseed/multistore#22).

<!-- Describe the high-level goals of the change -->

## How I did it

Updated multistore to ignore any keys returned by the object store
backend that has 0 bytes and ends with `/` (these keys are still
returned as `CommonPrefixes`, but not `Objects`)

<!-- Discuss the implementation strategy and considerations made -->

## How to test it

This backend is currently enabled as the backend of
https://source-cooperative-git-feat-data-connection-admin-radiantearth.vercel.app.
As such, compare the following:

* Before: https://staging.source.coop/harvard-lil/staging-gov-data
* After:
https://source-cooperative-git-feat-data-connection-admin-radiantearth.vercel.app/harvard-lil/staging-gov-data

You'll see an empty `staging-gov-data` in the before, but no longer in
the after.

<!-- Inform the reviewer how they can validate that these changes work
-->

## PR Checklist

- [ ] This PR has **no** breaking changes.
- [ ] I have updated or added new tests to cover the changes in this PR.
- [ ] This PR affects the [Source Cooperative Frontend &
API](https://github.com/source-cooperative/source.coop),
      and I have opened issue/PR #XXX to track the change.

## Related Issues

Resolves source-cooperative/source.coop#245

Author: alukach

Cargo.lock

@@ -20,8 +20,8 @@ path = "tests/pagination.rs"
 
 [dependencies]
 # Multistore
-multistore = { version = "0.3.0", features = ["azure"] }
-multistore-path-mapping = "0.3.0"
+multistore = { version = "0.3.1", features = ["azure"] }
+multistore-path-mapping = "0.3.1"
 
 # Serialization
 serde = { version = "1", features = ["derive"] }
@@ -36,7 +36,7 @@ tracing = "0.1"
 
 # Wasm-only dependencies (Cloudflare Workers runtime)
 [target.'cfg(target_arch = "wasm32")'.dependencies]
-multistore-cf-workers = { version = "0.3.0", features = ["azure"] }
+multistore-cf-workers = { version = "0.3.1", features = ["azure"] }
 futures = "0.3"
 wasm-bindgen-futures = "0.4"
 console_error_panic_hook = "0.1"