Make heimdall timeout configurable

[manav2401]

Rationale

There's a hard coded timeout of 5s in bor when we fetch data from heimdall via api. John suggested to make it configurable as public endpoints might take more time (although it's not recommended to use public endpoints).

bor/consensus/bor/heimdall/client.go

Lines 36 to 37 in 186deec

	apiHeimdallTimeout = 5 * time.Second
	retryCall = 5 * time.Second

Implementation

Add a new flag and pass it across bor to configure it.

[marcello33]

PR to be reviewed and possibly merged.