Support specifying a timezone manually with strptime parser

[guilload]

For instance:

name: timestamp
type: datetime
description: Time at which the event was emitted
input_formats:
  - rfc3339
  - unix_timestamp
  - "%Y %m %d %H:%M:%S.%f %z"
  - strptime:
      format: "%Y %m %d %H:%M:%S.%f"
      default_timezone: America/New_York