Implement optional dense indexes

[patszt]

Creation and usage of optional dense index can be optional for a given type or field name.
It can be configured in vast.yaml under index in each rule.
By default the index is always created.

📝 Checklist

• All user-facing changes have changelog entries.
• The changes are reflected on vast.io, if necessary.
• The PR description contains instructions for the reviewer, if necessary.

🎯 Review Instructions

Starting with docs might be useful to grasp what the change is supposed to bring

[dominiklohmann]

I only looked at the docs so far; I'll leave the actual code review to the rest of the team.

[lava]

Looks great, and very good test coverage!

[dominiklohmann]

Thanks for updating the docs. Just some minor requests for improvements.