unify TxBuilder and TxBuilderRunner interfaces?

[michele-nuzzi]

to improve UX, we could unify the two classes.

The constructor would take either protocol params or IProvider

if IProvider is passed the getProtocolParameters method can be used

if protocol params are passed, all the async methods of TxBuilderRunner will throw an error.

Additonally the TxBuilder would benefit of the incremental methods exposed in the runner.

One approach for implementation would be to add a _buildArgs private property, that is updated with every TxBuilderRunner method

when calling buildSync if other arguments are passed, the private property _buildArgs and the argument to the method should be merged

buildSync should still be the only "final" way to build a tx.