message: feat(account-lib): supporting adding signatures to transactions
  with TSS, we'll need to add the final signature to a transaction
  Ticket:
diff: import BigNumber from 'bignumber.js';
  import { BaseCoin as CoinConfig } from '@bitgo/statics';
  -import { BaseAddress, BaseKey } from './iface';
  +import { BaseAddress, BaseKey, PublicKey } from './iface';
  import { BaseTransaction } from './baseTransaction';
  import { SigningError } from './errors';
  @@ -52,6 +52,16 @@ export abstract class BaseTransactionBuilder {
  this.transaction = this.signImplementation(key);
  }
  + /**
  + * Adds a signature to the transaction.
  + *
  + * @param publicKey public key that produced the signature
  + * @param signature raw signature as a hex encoded Buffer
  + */
  + addSignature(publicKey: PublicKey, signature: Buffer): void {
  + throw new SigningError(`${this.coinName()} does not support adding signatures directly.`);
  + }
  +
  /**
  * Coin specific implementation of {@code sign}.
  *
  