Add hooks to allow overriding Helper field in Syncer

[bisgardo]

Fixes #422.

Motivation

For rosetta-cli check:data to do accounting correctly on the Concordium blockchain, it needs to rewrite account addresses into the same "normalized" alias.

Solution

Add (stateful)syncer options for overriding syncer.Options passed to syncer.New from outside the statefulsyncer.New.

Our internal fork of rosetta-cli is able to utilize this in https://github.com/Concordium/rosetta-cli/pull/1/files.

Open questions

There might be better ways to solve the underlying problem than patching block results, but I haven't been able to find one that works as reliably. So even if this is a bit brute-force'ish it's good enough for me.

[bisgardo]

Is anyone ever going to take a look at this? @jingweicb @shrimalmadhur @GeekArthur

[jingweicb]

Is anyone ever going to take a look at this? @jingweicb @shrimalmadhur @GeekArthur

@bisgardo sorry for making you waiting, will circle back later