[BUG] multisig PubKey bombay-11 migration return empty sub-PubKeys

[yun-yeo]

Describe the bug
When we migrate tequila-0004 exported genesis into bombay-11 genesis, cosmos@v040 migration is decoding tendermint/PubKeyMultisigThreshold to LegacyAminoPubkey,

type LegacyAminoPubKey struct {
	Threshold uint32       `protobuf:"varint,1,opt,name=threshold,proto3" json:"threshold,omitempty" yaml:"threshold"`
	PubKeys   []*types.Any `protobuf:"bytes,2,rep,name=public_keys,json=publicKeys,proto3" json:"public_keys,omitempty" yaml:"pubkeys"`
}

, which is registered in here and here.

However, the Any only supports proto style decoding. When we decode amino-json multisig PubKey, it only returns empty sub-PubKeys.

{
   "height":"5944561",
   "result":{
      "type":"core/Account",
      "value":{
         "address":"terra1gufrav46pnpwf03yu7xz76ylkmatsxtplrxnmc",
         "public_key":{
            "type":"tendermint/PubKeyMultisigThreshold",
            "value":{
               "threshold":"2",
               "pubkeys":[
                  
               ]
            }
         },
         "account_number":"55213",
         "sequence":"2"
      }
   }
}

To Reproduce
Migrate core@v0.4.6 state to core@v0.5.3

Context & versions
Make new LegacyAminoPubKey data type for migration and register this as tendermint/PubKeyMultisigThreshold codec to properly decode legacy multisig PubKey.

LegacyAminoPubKey struct {
   Threshold sdk.Int              `json:"threshold"`
   PubKeys   []cryptotypes.PubKey `json:"pubkeys"`
}

(if applicable) suggested solution
If you know the solution to the bug, let us know!