PR #26082 added account listing to OnSignerStartup but did not consider the case where a user has a large number of accounts which would be annoying to display.

This PR updates showAccounts() so that if there are more than 20 accounts available the user sees the first 20 displayed in the console followed by: First 20 accounts listed (N more available).
Co-authored-by: Martin Holst Swende <martin@swende.se>