Add a user defined source data transformer example for numaflow e2e test

[KeranYang]

Event Time Filter

This is a simple User Defined Function example which receives a message, applies the following data transformation, and returns the message.

Data Transformation

• If the message event time is before year 2022, drop the message.
• If it's within year 2022, update the key to "within_year_2022" and update the message event time to Jan 1st 2022.
• Otherwise(exclusively after year 2022), update the key to "after_year_2022" and update the message event time to Jan 1st 2023.

Signed-off-by: Keran Yang yangkr920208@gmail.com

[vigith]

Please make sure for images, the image name starts with mapT; we have a convention for the example image names, https://quay.io/organization/numaio