feat: typescript definitions for mysql2 lib

[rrhodes]

Closes #387.

Extending the AWS X-Ray MySQL library to support patching of MySQL2 connections, pools, and pool clusters.

Type definitions are derived for the most part from index.d.ts with thanks to @jj22ee for their valuable feedback on this PR.

There's scope to extend this library further to support mysql2/promise, allowing for usage of promises rather than the callbacks that this PR focuses on.

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

[jj22ee]

Hi @rrhodes. The failing workflow is unrelated to your PR (it's affecting other PRs as well). We will look into fixing this issue, which is also described in #688

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.56%. Comparing base (e28c1af) to head (3050e6d).
Report is 2 commits behind head on master.

Additional details and impacted files

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##           master     #676   +/-   ##
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  Coverage   83.56%   83.56%           
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  Files          36       36           
  Lines        1813     1813           
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  Hits         1515     1515           
  Misses        298      298           

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[jj22ee]

By default, we should also support mysql2.createConnection that returns a Connection. Our instrumentation does support a return value of a Promise<Connection>, but that requires supporting TS definitions of patched mysql2/promise rather than mysql2, which only the latter is supported by this PR.

[rrhodes]

I've started to de-scope this PR to support return of Connection only, rather than extending it to support Promise<Connection>: 8d4597f

[rrhodes]

Thanks @jj22ee for calling out the distinction between mysql2 and mysql2/promise, which I didn't quite appreciate on the outset.

I'd be happy to explore an extension of X-Ray's type definitions for the latter in future, but I'll keep this PR specific to mysql2 for the time being.