[pkg/stanza] Add mapstructure support to syslog_input operator configuration

[djaglowski]

Dependent on open-telemetry/opentelemetry-log-collection#13.

This will allow explicit embedding in the collector's syslogreceiver. (See #2231)

• Should not affect functionality of configuration
• Ideally would not remove support for yaml unmarshaling
• This change should perhaps be preceded by enhanced test cases on the syslog_input configuration

[djaglowski]

Closed by #13294