Change Koa openSegment middleware declaration return type to match expected Koa.Middleware type

[carolabadeer]

Issue #, if available: This PR fixes issue #522

Description of changes:
The issue above mentions a type error that occurs when using the opensegment middleware in koa and typescript due to a mismatch between the expected return type, Koa.Middleware, and the type declared for the opensegment middleware, Function.

The type declaration is updated below to fix this issue and avoid this manual workaround:

app.use(XRayKoa.openSegment("test") as Koa.Middlware);

and allow for consistency in using this middleware in JS & TS.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov-commenter]

Codecov Report

Merging #525 (fc36d3b) into master (d2abfd3) will not change coverage.
The diff coverage is n/a.

❗ Current head fc36d3b differs from pull request most recent head cca16ef. Consider uploading reports for the commit cca16ef to get more accurate results

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  Hits         1446     1446           
  Misses        310      310           

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[willarmiros]

lgtm