wormchain: add slashing params vaa

[joelsmith-2019]

This PR adds a new Wormhole core governance action payload to Wormchain, allowing guardians to update the slashing parameters. I'm unsure where I should include documentation for how the payload is structured. If theres a specific README, please let me know and I'll add some documentation on it.

// Core Wormhole Module
Module [32]byte = "Core"

// Action index (6 for Slashing Params Update)
Action uint16 = 6

Chain uint16

// Signed Blocks Window (big-endian uint64)
SignedBlocksWindow uint64

// Min Signed Per Window (big-endian uint64)
MinSignedPerWindow uint64

// Downtime Jail Duration (big-endian uint64)
DowntimeJailDuration uint64

// Slash Fraction DoubleSign (big-endian uint64)
SlashFractionDoubleSign uint64

// Slash Fraction Downtime (big-endian uint64)
SlashFractionDowntime uint64

Ref to Cosmos SDK Slashing Params: Params

[bruce-riley]

I only reviewed the sdk stuff.