services/horizon: Add muxed account details to transactions and change schema

[2opremio]

Addresses part of #3591

It:

• Adds muxed account details to transactions (account source and fee account source)
• Adds new DB table columns for muxed accounts. This changes the approach used for effects and operations, for which we previously abused the details field to include the muxed account information.
• Adds an integration test to make sure the new Muxed Account details work end-to-end.

TODO: add an integration test (which is currently WIP and will come shortly in a separate PR).