Decentralize message type generation

We want to support many different message type, some from the Cosmos SDK and some from custom blockchains that are not standardized. The user of CosmJS should have the most common message types ready to be used shipped with this library. But it should also be possible to add new message types from the caller code.

Some thoughts that might help getting there:

1. Protojs allow dynamically adding .proto files: https://github.com/protobufjs/protobuf.js/blob/master/README.md#using-proto-files
2. Maybe we can utilize JSON descriptors or reflection to dynamically add types
3. The library @node-a-team/ts-amino uses decorators to add encoding information to TypeScript types as shown here: https://github.com/chainapsis/cosmosjs/blob/master/src/common/stdTx.ts. Maybe we can use decorators as well to start with a TypeScript class and annotate it with all the necessary runtime information, like:

@CosmosMessage("cosmos.bank.MsgSend") // adds this type to a registry using the string as the `type_url`
export class MsgSend {
  @Field.Bytes(1 /* protobuf field number goes here*/)
  public fromAddress: Uint8Array;

  @Field.Bytes(2, { fieldName: "to_address" /* runtime field name for protobuf; probably irrelevant since not encoded into the proto bytes*/  })
  public toAddress: Uint8Array;

  @Field.Repeated(3, { type: Type.Defined }) // don't know what this does but if it worked for Amino, it can probably be adapted
  public amount: Coin[];
}

[willclarktech]

Good table of tradeoffs here: https://github.com/protobufjs/protobuf.js/blob/7bacfc8/README.md#reflection-vs-static-code

[willclarktech]

Closed in #221