Pass relay balance guard value using cli parameter

[svyatonik]

Right now values for balance guards are hardcoded (e.g.

parity-bridges-common/relays/bin-substrate/src/chains/kusama_headers_to_polkadot.rs

Line 28 in 7e31834

pub(crate) const MAXIMAL_BALANCE_DECREASE_PER_DAY: bp_polkadot::Balance = 30_000_000_000;

,

parity-bridges-common/relays/bin-substrate/src/chains/polkadot_headers_to_kusama.rs

Line 28 in 7e31834

pub(crate) const MAXIMAL_BALANCE_DECREASE_PER_DAY: bp_polkadot::Balance = 100_000_000_000;

and so on). But actually this value is different in different relay modes. Some values are now computed for relay-headers mode, when relay is (roughly) syncing all headers. Some - for the obsolete version of --only-mandatory-headers mode, when relay has been relaying mandatory headers and paid for it (now: submitting mandatory headers is free). But e.g. when we start relay in --relay-headers-and-messages mode we don't know in advance how many tokens relay may lose/gain. So for every development it makes sense to customize this value.

related to #1300

[serban300]

Might also be related to #2416