Implement price publishing

[0x-r4bbit]

Implement the publish_price instruction. Computes the TWAP over a requested window from two Observation Account snapshots and writes the result to the Price Account.

twap_tick = (tick_cumulative[t2] - tick_cumulative[t1]) / (t2 - t1)
price = 1.0001 ^ twap_tick

This will need #117

• Callable by anyone
• t2 is always the most recent observation, t1 is the snapshot closest to now - window_length
• Rejects zero and negative prices before writing to the Price Account
• If the ring buffer does not contain a snapshot old enough to satisfy the requested window, the instruction returns an error rather than
  publishing a price based on a shorter window than requested
• The window length is a parameter — different consumers can request different windows. This means multiple Price Accounts can exist for
  the same (base_asset, quote_asset) pair, one per window length. The Price Account PDA is derived from (oracle_program_id, base_asset,
  quote_asset, window_length, source_id)