feat(base): loadMarketsAndSignIn

[ttodua]

there are many cases of non-optimal flow like in all private methods (related exchanges):

async fetchXyz () {
   await this.loadMarkets();
   await this.signIn ();

}

so we would call just loadMarketsAndSignIn

[carlosmiei]

@ttodua We can add it but be careful, when for instance we need to sign in to call fetchCurrencies or any other private endpoint because the order is not assured