(2) Settler: Intent lifecycle

[GuidoDipietro]

Adds Settler program, with instructions pertinent to the lifecycle of an Intent PDA.

Includes Rust program, Typescript SDK, and Typescript tests.

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Instructions

Following are instructions for creation, extension, and deletion of an Intent.
Additionally, the initialize and set_paused_state instructions were added to manage the Settler program's global state.

initialize

Sets global state, defining the Whitelist program to be used and the paused state.
This program doesn't have an admin.

set_paused_state

Pauses or unpauses program. The admin of the defined Whitelist program can execute this action.

create_intent

Initializes an Intent PDA with all its fields.
NOTE: Some of these fields might not be necessary to have on-chain (or a hash might be enough). I have simply added them all until the Settler's first iteration is done and I can remove what is not necessary, as it is easier to remove than to add here. For now, we are assuming that we need all the fields.
Technical debt: Check that the Intent hash is correct.

extend_intent

Adds more data, max_fees, or events to an Intent PDA in case we ran out of space in the create_intent call.
Solana transactions have a limited size of 1232 bytes, so we can only pass in that many bytes (even less since there's other overhead) as instruction parameters. For this reason, Intents have an intermediate state with is_final=false where they can still be extended by the intent_creator with more data added in further transactions, non-atomically.

claim_stale_intent

If an Intent expired (deadline is in the past), the intent_creator can close the PDA and reclaim the rent, as this account is now useless.

State

Intent

PDA defining an Intent with all its fields.
NOTE: This account will be reduced in a future iteration, with fields that aren't necessary on-chain being removed.

FulfilledIntent

A minimal on-chain proof that an Intent existed and was executed, to save as much rent as possible.
When an Intent is executed, it is closed, rent goes back to the creator, and this PDA is initialized in its place.

SettlerSettings

Global state of the program.

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NOTE: Some files might have content from future PRs given this series of PRs come from a larger PR that was split (#41). I have tried to keep this to a minimum but bear this in mind.

[PedroAraoz]

I would update the imports to make them a bit shorter and reduce verbosity
from: Box<Account<'info, crate::whitelist::accounts::GlobalSettings>>,
to: Box<Account<'info, WhitelistSettings>>,
the same with the errors as we discussed offline.

[PedroAraoz]

are we doing something with this account?

[GuidoDipietro]

We check this in a future instruction to validate that a given intent has not already been executed on-chain.
I.e., this is our on-chain proof that a given intent was fulfilled.