[Feature Request] Make dependency on PGV opt-in

[ash2k]

Feature description:

Make dependency on PGV opt-in.

Problem it solves or use case:

To reduce the number of dependencies, make it possible to consume the library without depending on PGV when it's not needed.

Proposed implementation or solution:

Make legacy a separate module?

Contribution:

I'm not willing to contribute, sorry.

Examples or references:

Additional context:

[rodaine]

With the v1 release of protovalidate (and the runtime libraries) we will remove the legacy support package entirely.