[fix] Missing inheritance preventing `Batch` object use in parameter sets

[nwithan8]

Description

• Fix Batch class not inheriting IBatchParameter, preventing a Batch object from being used in a parameter set. Ex:

  Batch batch = await client.Batch.Retrieve("batch_123");
  EasyPost.Parameters.Pickup.Create parameters = new()
  {
  Batch = batch, // compile error due to mismatch type
  Instructions = "Special pickup instructions",
  };

• Prep for v6.5.1 release

Testing

• Test snippet above now compiles

Pull Request Type

Please select the option(s) that are relevant to this PR.

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] Improvement (fixing a typo, updating readme, renaming a variable name, etc)