The reason that Unity does not provide any specific mechanism to limit download requests is that the Unity Engine powers your client-side application. It does not run your asset-serving server, and any protections against denial of service attacks must be implemented server-side.
Imagine if the Unity Engine did provide some way to for the client sign an asset bundle request. As the information security mantra goes, the client is in the hands of the enemy. Anyone with a copy of your game (either a paying customer, or someone who has obtained a copy illegally) could decompile it, extract the request-signing code, and use it to build a fake client that spams your server with flawlessly forged requests. Trying to defend against this in the client is a losing game.
(This is also the reason why you should consider any game assets you package or download into the client to be public information - you can't keep a secret in something you want every player of your game to be able to use)
So, if malicious downloads are a concern for your use case, you'll need to use server-side solutions. Content distribution networks exist to alleviate the load of file serving from servers, and detect/block denial of service attacks, or you can implement your own customized protections.
One possibility would be for a client to first send a conventional GET request asking for permission to download the asset. After validating the request (say, rate-limiting requests by IP, or checking the user's login credentials if you want to restrict the downloads to specific user accounts in your own database), the server can reply with a one-off URI unique to that request. Then your server can respond to requests to that special URI by serving the asset bundle, up to a certain number of download attempts or short deadline, whichever is reached first.
This gives you the flexibility to implement whatever validation approach you deem appropriate for your threat model, entirely on the server side where it's out of the reach of would-be attackers to forge or manipulate.