feat: Allow libp2p user agent to be overriden

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Related Issues

Partially addresses: filecoin-project/lily#692

Proposed Changes

1. Adding a special option for configuring the UserAgent
2. Adding a function callable by 3rd parties to override the user agent

Additional Info

Checklist

Before you mark the PR ready for review, please make sure that:

• Commits have a clear commit message.
• PR title is in the form of of <PR type>: <area>: <change being made>
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• Tests exist for new functionality or change in behavior
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cc @frrist