Move `ConversionRate` out of `account::oracle` into a shared module

As discussed in this comment: https://github.com/0xMiden/protocol/pull/3561#discussion_r3834672363

`ConversionRate` is in `crates/miden-standards/src/account/oracle/types.rs`, inside the price oracle component's module. However, it is not oracle-specific, a rate is a general-purpose value type, and the fee standard already applies the same concept through `fee::convert_amount`.

So, we should Move `ConversionRate` out of `account::oracle` into a module that both the "oracle" and the "fee standard" can depend on.