Allow selecting an exact version of a port inside vcpkg.json instead of using overrides

[aminya]

Currently, vcpkg only allows specifying the minimum version, and to give an exact version, a separate "override" entry should be used. This is very unintuitive and verbose.

Is your feature request related to a problem? Please describe.
Specifying an exact version is hard in the current manifest format

Proposed solution
Allow adding version entry, which follows the exact version.

Describe alternatives you've considered
Using the overrides entry of vcpkg.json

Additional context
Only specifying the minimum version has many issues. For example, whenever the library updates, the whole thing should be rebuilt. Let's say libjpeg is updated, then the whole opencv needs to be rebuilt, which takes a lot of time.