feat: add client method to estimate recent blockhash

[lpahlavi]

(XC-317): Add client method to estimate recent blockhash by repeatedly fetching getSlot and getBlock until either the maximum number of retries is reached, or a blockhash is obtained.

The new method is tested in end-to-end tests and solana_test_validator.rs, however further integration tests will follow in another PR due to the mock runtime currently not supporting mocking responses for sequential calls.

[gregorydemay]

thanks a lot for this PR @lpahlavi ! Only some minor comments from my side.

[gregorydemay]

nit: continue appears quite often and some times it's easy to miss that one is missing. I would rewrite the code a bit to be safer. Something like

pub async fn send(self) -> Result<Hash, Vec<EstimateRecentBlockhashError>> {
        let mut errors = Vec::with_capacity(self.num_tries.into());
        while errors.len() <= self.num_tries.into() {
            match self.get_slot_then_get_block().await {
                Ok(hash) => return Ok(hash),
                Err(e) => {
                    errors.push(e);
                    continue;
                }
            }
        }
        Err(errors)
    }

    async fn get_slot_then_get_block(&self) -> Result<Hash, EstimateRecentBlockhashError> {
        let slot = self.get_slot().await?;
        match self.get_block(slot).await? {
            Some(block) => match Hash::from_str(&block.blockhash) {
                Ok(blockhash) => return Ok(blockhash),
                Err(e) => Err(EstimateRecentBlockhashError::GetBlockRpcError(
                    RpcError::from(e),
                )),
            },
            None => Err(EstimateRecentBlockhashError::MissingBlock(slot)),
        }
    }

    async fn get_slot(&self) -> Result<Slot, EstimateRecentBlockhashError> {
        todo!()
    }

    async fn get_block(
        &self,
        slot: Slot,
    ) -> Result<Option<UiConfirmedBlock>, EstimateRecentBlockhashError> {
        todo!()
    }

[lpahlavi]

Good idea! Done.

[gregorydemay]

Very cool PR, thanks a lot for all the improvements @lpahlavi !