Add mapstructure support to parser configurations

[djaglowski]

Similar to #14, open-telemetry/opentelemetry-collector-contrib#10287, #16, #17, but will have a different effect on the collector receivers.

Currently, the collector unmarshals these configs into an []map[string]interface{}, and then uses yaml marshaling. Since we don't know which operators will be included and in what order, this might not have as much of a benefit as the other tickets, but there is likely a better design than the current.

General guidelines:

• 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 parser configurations