Feat: Server cost model

[hopeyen]

To add price matching in subfile exchanges,

1. Start with adding a CLI config --price-per-byte to ServerArgs.
   The price is not stored in a database or subfile-specific at the moment. There may be a case where the bytes in a subfile is more valuable than another, then we can consider a more complex management.

2. Add a /cost endpoint that returns the price per byte for client/third-party price matching

3. When receiving paid queries, parse for receipts and condition on the receipt value where $\text{price per byte} * \text{bytes range} \leq \text{receipt value}$

4. Add a admin/cost endpoint for adjusting price_per_byte on the fly, taking a method set_price and a param price_per_byte to replace the previous price.

Additionally consider if indexer_framework can take care of cost models

[hopeyen]

Done 1,2,4 in 650b741
3 left for future when receipt is constructed