Make preamble in vector query prompt customisable

[igor-2lemetry]

• I have read and agree to the contributing guidelines.

Is your feature request related to a problem? Please describe.
This is a good widely used approach to have a preamble in the very beginning to let LLM to take up a role, before providing the rest of data like user query, contex, etc. It is great to have a default value for it, but many cases and advances users it may matter. So, I believe it is important to make it customisable.

Describe the solution you'd like
Provide an ability to overwrite default value by adding new attribute to the TextQueryTask.

Describe alternatives you've considered
Alternative is to accept the fate and use the default value.

Additional context
No additional context here.