Fix transaction metadata type

[olajiredominic]

PR updates transaction metadata type to Metadata

Root Cause

Error when transaction.Verify(...) method is called

1 error(s) decoding: * 'metadata' expected type 'string', got unconvertible type 'map[string]interface {}

[elvis-chuks]

@rpip any chance we can get this merged in?

[odetolakehinde]

this thing needs to be merged.

@rpip pls help look into it.