Add config to exclude function from Datadog

[nholbrook]

What does this PR do?

This PR adds a config option that allows users to specify functions to be excluded from Datadog.

Motivation

Currently, there is no way to prevent Datadog from updating certain functions. One example of a scenario where this is a problem is when using this plugin with serverless-plugin-warmup. Datadog will update the handler of the warmer function created by this plugin, which prevents it from working.

Testing Guidelines

I added an additional test case and updated all associated test cases.

Additional Notes

This issue is documented here.

Types of changes

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

Check all that apply

• 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

[DarcyRaynerDD]

Hey @nholbrook thanks for the PR. This is looking good, and I think we will accept the feature for the next release, but for some reason the github actions to verify the branch weren't triggered. I'm going to pull this branch into the repo, and recreate the PR.

[nholbrook]

Closing because @DarcyRaynerDD recreated this PR (#87) due to issue with GitHub actions.