Add the vkpay package: verify a VK Mini Apps payment ("голоса") callback
signature — MD5 (mandated by VK's payment protocol) of the sig-excluded
parameters, sorted by name and concatenated key=value, with the app secret
appended; case-insensitive over the hex digest. Unit-tested (valid / tampered /
wrong-secret / missing). First piece of the VK rail; the two-phase callback
handler, the VK order branch and the client bridge follow.
Ilia Denisov
3e0763463f