Add new text-followup handler to use the output of text-followup preprocessor

[VenissaCarolQuadros]

This handler responds to followup user queries using the output of the text-followup preprocessor (#947).

This handler has been tested for different queries using the command

jq '.followup.query = "<MY_QUERY>"' /home/venicq/follow-up.json | /var/docker/image/bin/sendimagereq /dev/stdin https://unicorn.cim.mcgill.ca/image/render/

Here are some examples:

1. Response to generic query

Query: What are the people wearing?
Response: The women appear to be dressed in business casual attire, likely for an office setting.

2. Logs and response when text-followup preprocessor is down/ returns invalid JSON (simulated by modifying prompt)

Logs: DEBUG:root:Text-followup preprocessor not found
Response: "renderings":[]

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