stick to go's idea of exporting structs and defining interfaces at consumers in polygon/p2p,sync,heimdall packages

[taratorio]

we have a bunch of constructors in polygon/p2p,sync,heimdall packages that return interfaces this goes against go's idea of always returning/exporting concrete structs and defining interfaces at consumers

• change all constructors in those new packages to return pointers to concrete structs
• define the interfaces at the consuming components/packages as private interfaces (lowercase)
• should not change the logic simply a stylistical change to align us better with the spirit of go