Need a way for public_key_to_string() to output in EOSxxxx format

Scatter does not support the new key format yet (PUB_K1_xxx), so there should be a way to export the key strings in legacy format.

Probably a solution would be to move the function into `abieos.cpp` and to define a global variable that enables compatibility format.

Start by locating public_key_to_string() and reviewing the key-format handling in the repository, including the proposed abieos.cpp location. Determine how compatibility mode should be selected and verify that public keys can be exported in the legacy EOSxxxx format as well as PUB_K1_xxx.

Written by the indexing model from the issue text.