[ecash-wallet] Export the BuiltAction class so it is available for ecash-agora take method

Summary:
Ref D18833

Isolating this here as this must be built and published for ecash-agora to get it in its CI build / publish, so we need to land this first

Test Plan: npm test

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18840