feat(rag:etl): Add custom template support to KeywordMetadataEnricher

[YunKuiLu]

• Support the use of custom templates
• Adding builder pattern
• Adding unit tests
• Adjusted the relevant documents

If this PR is merged, it will also solve the following issues：
#2698

[torakiki]

To add on this, the current documentation (1.0.0) says

The keyword extraction prompt can be customized by modifying the KEYWORDS_TEMPLATE constant in the class.

but KEYWORDS_TEMPLATE is final so there isn't at the moment any way to change the template beside creating your own MyKeywordMetadataEnricher.
I needed to change the template because llama3.2 was returning "here are five keywords: bla bla" and I had to be more strict on the prompt adding something like "Return only the comma separated keywords"

[markpollack]

oops! great catch. Thanks @YunKuiLu

[markpollack]

Thanks for the PR. Thanks for the context @torakiki you should be good to go now.

merged in f3e9d04c3ffb852ab2198f77887b46e600c2dc5b

[YunKuiLu]

Hi @markpollack. I found that this PR is in the 1.0.x branch but not in master. Should we wait until the 1.0.1 release before merging it into master?

[ilayaperumalg]

Re-opening the PR as the commit f3e9d04c3ffb852ab2198f77887b46e600c2dc5b didn't make it to main yet.

[YunKuiLu]

Closing because it has been merged into main as c122fe1.