[ecash-lib] Move signatories to unique file

Summary:
This makes it easier to add new signatories which will be needed for multisig

No version bump as we do not plan to publish yet. This is part of a split-out effort to make diffs smaller and simplify review of multisig / PSBT implementation in ecash-lib.

Test Plan: npm test

Reviewers: #bitcoin_abc, tobias_ruck

Reviewed By: #bitcoin_abc, tobias_ruck

Differential Revision: https://reviews.bitcoinabc.org/D19711