Create a vcpkg overlay for cpprestsdk and libyaml

[florelis]

This PR adds a local port overlay so that we can have our own custom vcpkg ports. In particular we use it to:

1. Apply a patch to cpprestsdk for certificate pinning, without cloning the whole project
2. Apply a patch to libyaml for a security vulnerability, as there is no updated version with the fix

These ports are mostly a copy of the official ports, but with the patches we want applied. Unfortunately the original port recipes were already applying some patches, and we had to include them here because vcpkg provides no mechanism to apply a single additional patch on top of an existing port.

Notes for the future:

• We could potentially use the port overlay to create ports of the other libraries we have clones of, without having all their code checked in
• For libyaml, instead of applying a patch we could simply change the base commit to one that already has that change
• For libyaml, we could also try to contribute the patch to the official port

Microsoft Reviewers: Open in CodeFlow

[JohnMcPMS]

Did you mean to update the SFS Client entry? It appears to be a libyaml commit hash.

[florelis]

I did not. Fixed now to add libyaml