feat: add collection serialization to lp_dump

[dwong2708]

Description

Resolves #361
This PR adds support for serializing collections into the learning package dump file.

Key Changes

• Each collection is serialized as a TOML file within the dump folder.
• Added a prefetch_related query on the collections queryset to improve performance.
• When generating collection TOML files, related entities are included, but their versions are not.

Example Output

Filename: col1_06bb25.toml

[collection]
title = "Collection 1"
description = "Description of Collection 1"
created = 2025-09-03T18:56:57.524111Z

# ### Entities

[[entity]]
uuid = "4ebf2668-75e2-4e27-b339-33b45fd2a087"
can_stand_alone = true
key = "xblock.v1:problem:my_published_example"

[[entity]]
uuid = "a068659e-27ea-4495-82ad-9d0bf11d64da"
can_stand_alone = true
key = "xblock.v1:html:my_draft_example"

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[wgu-taylor-payne]

Looks good overall, Daniel. Here are some suggestions from a quick, high-level overview.

[ormsbee]

The entity information will already come out with the TOML files for each entity. The collection just needs to point to it using its identifiers, like containers will. So something like this would work:

 [collection]
title = "Collection 1"
key = "collection-1"
description = "Description of Collection 1"
created = 2025-09-03T17:50:59.565939Z
entities = [
    "key-for-entity-1",
    "key-for-entity-2",
    # etc.
]

[dwong2708]

Applied. Thanks Dave

[ormsbee]

A couple of minor requests.

[ormsbee]

Nit: You should be able to use extend() here instead of appending one key at a time:

entities_array.extend(entity_keys)

[dwong2708]

Nice. Applied. Thanks

[ormsbee]

For the sake of making exports consistent (and easier to diff), we should make the ordering deterministic. Please sort this list by key.

[dwong2708]

Done