fix: Kafka external stream virtual columns

[zliang-min]

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• Did you separate headers to a different section in existing community code base ?
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Please write user-readable short description of the changes:

closes #55
refs #175

Fixed virtual columns in Kafka external streams:

• _tp_time now always has a value
• _tp_append_time is set only when message timestamp is an append time
• all time columns has DateTime64 type
• if a virtual column is explicitly defined an a pyshcial column, it should be respected

[jovezhong]

(Jove Github Bot) added it to the current sprint.

[jovezhong]

(Jove Github Bot) moved this ticket out of the GitHub project(up to 1200 tickets for one project).