Add sync mode p2p CLI flag

[weiihann]

Changes

• Add the p2p-sync-mode flag which supports either snap or full sync mode (default is full)
• Added Downloader component
  • The previous implementation includes the baseSyncer in the snapSyncer
  • This is too much of a coupling as both syncing mechanisms are different
  • We eventually need a method to switch between snapSync and baseSync, so a Downloader controls this syncing mechanism
  • Referenced this design from go-ethereum
• A couple of refactorings

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.

Please upload report for BASE (pnowosie/snap-sync-p2p@0455bf0). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
node/node.go	50.00%	0 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## pnowosie/snap-sync-p2p #2186 +/- ## ========================================================= Coverage ? 76.24% ========================================================= Files ? 103 Lines ? 9557 Branches ? 0 ========================================================= Hits ? 7287 Misses ? 1612 Partials ? 658 ```

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