Bug 20942: OpenAPI spec for /patrons/{patron_id}/account

Bug 20942: OpenAPI spec for /patrons/{patron_id}/account

This patch adds the OpenAPI spec for the following paths:
- /patrons/{patron_id}/account

It also adds object definitions for:
- balance
- account line

Account line is to be used on both /account/lines (when implemented)
and for embeding the outstanding lines in the balance endpoint
(/patrons/{patron_id}/account).

Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>