Use network instead of specifying tx version, chain ID and API URLs

[yknl]

Use a network object to specify tx version, chain ID and API URLs instead of setting them separately in the options object of transaction builders. Constants for Mainnet and Testnet should be made available to simplify transaction building.

Currently the options looks like

export interface TokenTransferOptions {
  nonce?: BigNum;
  version?: TransactionVersion;
  chainId?: ChainID;
  anchorMode?: AnchorMode;
  memo?: string;
  postConditionMode?: PostConditionMode;
  postConditions?: PostCondition[];
  transferFeeEstimateApiUrl: string;
}

Using networks:

export interface TokenTransferOptions {
  fee?: BigNum;
  nonce?: BigNum;
  network?: StacksNetwork;
  anchorMode?: AnchorMode;
  memo?: string;
  postConditionMode?: PostConditionMode;
  postConditions?: PostCondition[];
}