[jwt-cpp] Fix deps, install config

[dg0yt]

Resolves #41058.
Closes #35919.

CMake config was missing after the #36916 update because it didn't switch to a full installation. #35919 isn't finished, maybe because of the CMake build simply not working out of the box.

• The lib is header-only and flexible, however the CMake config needs choices for JSON and libcrypto. No features modeled at this point.
• picojson is the legacy choice for JSON by upstream and in the port.
• OpenSSL is the default choice for libcrypto by upstream.

The patch is the minimal way to make the public picojson header dependency available to users, by burning the picojson location into the jwt-cpp target properties. (JWT_EXTERNAL_PICOJSON would require a CMake package or a bigger patch.)