[bt][gap] Create ScoConnectionManager

Create ScoConnectionManager class that queues SCO connection requests,
establishes SCO connections, and owns open SCO connections for a
particular BR/EDR link. Currently only outbound requests are supported,
but requests for accepting inbound connections will be implemented next.

Add closed_cb parameter to ScoConnection ctor that is called when the
connection is closed/deactivated. This is used to notify the
ScoConnectionManager that a connection should be destroyed.

Fixed bug in ScoConnection where calling Close() on an un-activated
connection would not destroy the underlying hci::Connection.

Create HCI command structs for the Enhanced Setup Synchronous Connection
Command, the Enhanced Accept Synchronous Connection Request Command, and
the Reject Synchronous Connection Request Command.

Create HCI event struct for the Synchronous Connection Complete event.

Unmask Synchronous Connection Complete event.

Bug: 58458
Test: in bt-host-unittests
GAP_ScoConnectionManagerTest.*
GAP_ScoConnectionTest.*

Change-Id: I058ee937b8e4599c459d038bf5b1066cbe0e3c08
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/432937
Commit-Queue: Ben Lawson <benlawson@google.com>
Reviewed-by: Xo Wang <xow@google.com>
Testability-Review: Xo Wang <xow@google.com>