Run integration tests in GitHub

[nhinsch]

What does this PR do?

Run integration tests in GitHub actions, see corresponding PR in datadog-lambda-python for more information.

Also:

• Remove obsolete Gopkg.toml file
• Bump go directive in go.mod to 1.13. The integration tests require this function which requires Go 1.13.

Testing Guidelines

Integration tests run and pass in GitHub.

Types of changes

• Bug fix
• New feature
• Breaking change
• Misc (docs, refactoring, dependency upgrade, etc.)

Checklist

• This PR's description is comprehensive
• This PR contains breaking changes that are documented in the description
• This PR introduces new APIs or parameters that are documented and unlikely to change in the foreseeable future
• This PR impacts documentation, and it has been updated (or a ticket has been logged)
• This PR's changes are covered by the automated tests
• This PR collects user input/sensitive content into Datadog

[codecov-io]

Codecov Report

Merging #66 (3a2ceb0) into main (b45a6b5) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main      #66   +/-   ##
=======================================
  Coverage   81.77%   81.77%           
=======================================
  Files          12       12           
  Lines         631      631           
=======================================
  Hits          516      516           
  Misses         93       93           
  Partials       22       22           

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