[vpdq] Fix macOS build

[ianwal]

Summary

Clang doesn't like the forward declaration of the templates, so move them to the header file where they should belong.

The current way might actually be ill-formed, but GCC is pretty lax about template stuff in my experience so that's why it may be working.

Changes

• Fix Clang link error

• Fix deprecated warning for AVCodecContext::frame_number. This warning is present on the version of libav libraries that ship with Ubuntu 24.04.

• Remove unnecessary std::bind for spawning hashing threads in VpdqHasher ctor

  • This is not required to fix the build but generally much better practice. Since we're touching this area I made this change now.

  • This is the only change to the code while moving from hasher.cpp to hasher.h

  • See Herb Sutter's recommendation for this Add rule about preferring lambdas to std::bind isocpp/CppCoreGuidelines#1796

Test Plan

Local build and CI passes successfully.

Build and regtest pass on M2 Pro macos Sonoma with Clang with FFmpeg 7.0 installed with brew.

[Dcallies]

Apologies for the long delay, I've been traveling.

Thanks again for the fixes!