feat!: allow loading network-specific PriceAccount

[Reisen]

The PriceAccount structure has diverged between Solana and Pythnet, and the SDK currently only supports loading the format stored on Solana. To support loading different kinds of price account, this PR introduces a base type GenericPriceAccount that can be instantiated to the two different shapes as well as being extensible to future changes to price accounts.

The following steps were taken:

• Introduce GenericPriceAccount that can represent any underlying representation.
• Replace PriceAccount with an alias to GenericPriceAccount<32, ()> for backwards compatibility.
• Implements Deref on GenericPriceAccount so the underlying extended fields are ergonomic.
• Introduce SolanaPriceAccount and PythnetPriceAccount aliases.
• Introduce ::account_to_feed and ::account_info_to_feed over these types. These functions strictly speaking are PriceFeed constructors but are implemented as conversion functions namespaced under the types.
• Deprecate the solana specific load_* methods and re-implement the body with these new methods.
• Adds a small test to make sure the generic implementation and the old implementation are byte-equivalant.

CI also seems to be having trouble building with nightly so I've added toolchain files with the minimum required versions to all examples.