Basic snapsync algorithm

[pnowosie]

This is p2p/snap2 branch rebased onto current main.

During a work on p2p in snap-sync-p2p branch I've refactored this code in a better way - I will rebase it here and then we can review.

🚧 Needs to be cleaned & aligned to code standards
🚧 Need more tests to comply with codecov

[codecov]

Codecov Report

Attention: Patch coverage is 56.08263% with 574 lines in your changes missing coverage. Please review.

Project coverage is 75.47%. Comparing base (9c23832) to head (6185409).
Report is 34 commits behind head on main.

Files with missing lines	Patch %	Lines
sync/snapsyncer.go	55.59%	216 Missing and 50 partials ⚠️
core/state.go	21.65%	106 Missing and 17 partials ⚠️
sync/snap_server.go	64.58%	53 Missing and 32 partials ⚠️
blockchain/snap_server_interface.go	54.65%	29 Missing and 10 partials ⚠️
core/trie/snap_support.go	77.52%	10 Missing and 10 partials ⚠️
core/contract.go	0.00%	14 Missing ⚠️
blockchain/blockchain.go	74.35%	7 Missing and 3 partials ⚠️
core/trie/trie.go	71.42%	6 Missing and 2 partials ⚠️
core/trie/proof.go	0.00%	5 Missing ⚠️
db/pebble/db.go	75.00%	1 Missing and 1 partial ⚠️
... and 1 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2035      +/-   ##
==========================================
- Coverage   78.18%   75.47%   -2.71%     
==========================================
  Files         101      105       +4     
  Lines        9062    10362    +1300     
==========================================
+ Hits         7085     7821     +736     
- Misses       1344     1788     +444     
- Partials      633      753     +120     

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[pnowosie]

As agreed in #1828 - snapsync PR will include p2p
Follow up in #2120