use functional options pattern for constructors in polygon/p2p, polygon/sync, polygon/heimdall

[taratorio]

Do after: https://github.com/ledgerwatch/erigon/issues/11031

There are a bunch of private constructors in:

• polygon/p2p
• polygon/sync
• potentially others within the polygon/ package

That exist only because we need to pass different values than the defaults for tests, e.g:

• newBlockDownloader
• (scan the dirs for more)

This can be improved by using the functional options pattern: https://github.com/uber-go/guide/blob/master/style.md#functional-options.