Support LaunchDarkly anonymous users shared key

[chandlervdw]

Support LaunchDarkly key override

This supports options.integrations.LaunchDarkly.key as an override of the key field sent to the LaunchDarkly JS API. This allows for anonymous users to have a shared key so that the autogenerated Rudderstack ID for anonymous users is not sent to LaunchDarkly, and therefore all anonymous users share the same key. This is an additive change.
LaunchDarkly JS SDK key API

Type of change

• Bug fix (non-breaking change that fixes an issue)

Related issues

Fix #569
This originated with #570 but that was branched off of develop. This branches from v1-staging.

Checklists

Development

• Lint rules pass locally
• The code changed/added as part of this pull request has been covered with tests
• All tests related to the changed code pass in development

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• "Ready for review" label attached to the PR and reviewers mentioned in a comment
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