Implement passenger/vehicle handling when spawning entities

[Insprill]

Description

Uncomments the passenger/vehicle handling code when spawning entities. To properly support this, EntityMock#getIndirectPassengers has been added.

Another minor change that is a little small for it's own PR is passing the proper SpawnReason when dropping items.

Checklist

The following items should be checked before the pull request can be merged.

• [x] Unit tests added.
• [x] Code follows existing code format.

Info on creating a pull request

• Make sure that unit tests are added which test the relevant changes.
• Make sure that the changes follow the existing code format.

For maintainer

When a PR is approved, the maintainer should label this PR with release/*.

• If the PR fixes a bug in MockBukkit, update the patch version. (if the current version is 0.4.1, the new version should be 0.4.2)
• If the PR adds a new feature to MockBukkit, update minor version. (if the current version is 0.4.1, the new version should be 0.5.0)

Note that a PR that fixes an UnimplementedOperationException should be considered a new version and not a bugfix.

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[thelooter]

@Insprill can this be merged

[sonarqubecloud[bot]]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
0.0% Duplication