feat: calculate price for rate limits

[danisharora099]

Description

We are now offering two rate limit options to the users:

• standard: 300 messages/epoch
• max: 600 messages/epoch

With these options, the quantitative difference is not clear in terms of the token spend.

User Story

As a user, as I select a particular rate limit, I should be able to know how much token spend selecting a rate limit implies

Proposed Solution / Feature Design

Use PriceCalculator.calculate(rateLimit) in @waku/rln

Notes

[weboko]

Moved to todo. Will prioritize in accordance with next deliverables