build: move oss required to build conhost out of dep/ (#5451)

This change is necessary as the dep/ folder is not synced into the
Windows source tree.

I've also added a build rule producing a lib for {fmt}.

This will be required for our next OS ingestion.

oss/dynamic_bitset/LICENSE

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+MIT License
+
+Copyright (c) 2019 Maxime Pinard
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.

oss/dynamic_bitset/MAINTAINER_README.md

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+### Notes for Future Maintainers
+
+This was originally imported by @miniksa in March 2020.
+
+The provenance information (where it came from and which commit) is stored in the file `cgmanifest.json` in the same directory as this readme.
+Please update the provenance information in that file when ingesting an updated version of the dependent library.
+That provenance file is automatically read and inventoried by Microsoft systems to ensure compliance with appropiate governance standards.
+
+## What should be done to update this in the future?
+
+1. Go to pinam45/dynamic_bitset repository on GitHub.
+2. Take the entire contents of the include directory wholesale and drop it in the root directory here.
+3. Don't change anything about it.
+4. Validate that the license in the root of the repository didn't change and update it if so. It is sitting in the same directory as this readme. 
+   If it changed dramatically, ensure that it is still compatible with our license scheme. Also update the NOTICE file in the root of our repository to declare the third-party usage.
+5. Submit the pull.
+

oss/dynamic_bitset/cgmanifest.json

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+{"Registrations":[ 
+    {
+      "component": { 
+       "type": "git", 
+       "git": { 
+         "repositoryUrl": "https://github.com/pinam45/dynamic_bitset", 
+         "commitHash": "00f2d066ce9deebf28b006636150e5a882beb83f" 
+         }
+       }
+    }
+ ],
+ "Version": 1
+}