Extended price messages

[ckoopmann]

Extends the message format that will be sent to the server by the following fields:

1. interval_inclusion_messages: A list of signed message indicating all prices that are within a given interval
2. value_message: A single message containing the "exact" price value
3. validator_public_key: For signature validation on the server

The value_message and each element in interval_inclusion_messages is signed individually and includes the slot_number to avoid "replay attacks".

The values to which to attest in interval_inclusion_messages include all values within a relative range of +/- INTERVAL_SIZE_BASIS_POINTS and a precision of INTERVAL_STEP_DECIMALS.
So for current_price = 1800, INTERVAL_SIZE_BASIS_POINTS=100 (0.1%), INTERVAL_STEP_DECIMALS=2 (precision of 1 usd-cent) it would be ca. 1800 * 0.001 * 100 * 2 = 720 entries.

For the rust type definitions see here

Additional changes:

1. Switch from the protocol_labs bls library to the one from lighthouse. (which I factored out into a separate repo to be able to install it without the rest of lighthouse) - for better serde deserialization and to avoid apparent issues with signature aggregation in the former
2. Add a JsonFileMessageBroadcaster to write the messages to individual Json files in test_files for better inspection.

closes #3

[alextes]

I thought I left a comment about testing somewhere but can't find it. At the risk of repeating myself: I vaguely recall you mentioned you are all about testing, so where are all the tests 😄 ?

This is getting to a point where a bunch of testing seems healthy to me. Especially considering the fact we want this to be highly reliable. What is your thinking regarding testing?

[ckoopmann]

I thought I left a comment about testing somewhere but can't find it. At the risk of repeating myself: I vaguely recall you mentioned you are all about testing, so where are all the tests 😄 ?

This is getting to a point where a bunch of testing seems healthy to me. Especially considering the fact we want this to be highly reliable. What is your thinking regarding testing?

You are absolutely right, I should have done more testing.
I added some basic unit tests now for the components which are somewhat functional (price_provider, message_generator and signature_provider) as well as a very basic "integration test" where I mock out the gofer response with a static json file.

I didn't add unittests for components that are only placeholders / mocks and I suggest adding tests there when we replace those with the actual components. (message_broadcaster and slot_provider).