perforce_installer - for use as a build_requirement

[solvingj]

Many enterprise organizations use perforce internally for SCM. For these organizations, the p4 executable will likely be an integral part of their recipes source() methods.

In many cases, it will likely be substantially preferable to be able to retrieve perforce via a Conan package as a build_requires rather than installing using the system package manager or other mechanism.

Posting this here in part to get feedback from other conan users to see how many are using perforce today.