Add support to update price data, time stamps and valid slots

[hiddengearz]

Additionally, it would be cool to have some function to update oracle as a utility function in the banks_client, so you can modify price data, timestamps, valid slots between tests.

Originally posted by @f44z in #6 (review)

[hiddengearz]

In #8 I added an update_pyth_oracle function to ProgramTestContext that uses ProgramTestContext.set_account to overwrite the existing accounts data with new data. This isn't available with the rpc_client afaik.

In the future I plan to add support to clone accounts/programs from main-net. if that's doable we should be able to clone/cache the pyth program and then hopefully reverse engineer it's update function so we can use it with the rpc_client.