Simplify TransactionSummary type

[izri16]

Based on @xdzurman comment:

Now we have:

export type TransactionSummary = {
  type: TxType
  fee: Lovelace
  plan: TxPlan
} & (
  | SendTransactionSummary
  | WithdrawTransactionSummary
  | DelegateTransactionSummary
  | DeregisterStakingKeyTransactionSummary
)

and every sub-summary is an extension of that, perhaps I'd opt for having only the type, fee, plan in the bare default TransactionSummary and the more specific plans would be defined like SendTransactionSummary = TransactionSummary & {whatever...} and use just that, because there are multiple places in the code where this is handled very freestyle-like.