feat: re-export message interface

[nicholasgriffintn]

Resolves #384

Description:

It has been suggested that the package would be easier to use for developers if the library re-exported the AWS interface Message, so that they don't have to import it from the AWS library directly.

Type of change:

• 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 change)

Why is this change required?:

This PR re-exports the interface from the types file and also changes how we use the interface internally to also use that export.

Code changes:

• Changed the type exports to export at the end of the file rather than individually (this allows us to do the next item)
• Re export the Message type from the AWS SDK

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[codeclimate]

Code Climate has analyzed commit 2445f99 and detected 0 issues on this pull request.

The test coverage on the diff in this pull request is 100.0% (80% is the threshold).

This pull request will bring the total coverage in the repository to 94.7% (0.0% change).

View more on Code Climate.

[nicholasgriffintn]

So the problem with this is that the docs are blank, this means that our typedoc links to the type but nothing is displayed.