Add convenient GossipSync variant constructors

[wpaulino]

These constructors fill in the missing types for each variant so that users don't have to turbofish them manually.

I verified that we could use all of these within ldk-sample without needing to turbofish.

Fixes #1594.

[codecov-commenter]

Codecov Report

Merging #1618 (f3cb61f) into main (f3d5b94) will decrease coverage by 0.25%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #1618      +/-   ##
==========================================
- Coverage   91.07%   90.81%   -0.26%     
==========================================
  Files          80       80              
  Lines       44128    44645     +517     
  Branches    44128    44645     +517     
==========================================
+ Hits        40190    40546     +356     
- Misses       3938     4099     +161     

Impacted Files	Coverage Δ
lightning-background-processor/src/lib.rs	95.20% <ø> (ø)
lightning-invoice/src/payment.rs	87.76% <0.00%> (-5.15%)	⬇️
lightning/src/util/events.rs	39.25% <0.00%> (-2.74%)	⬇️
lightning/src/routing/scoring.rs	96.10% <0.00%> (-0.71%)	⬇️
lightning/src/routing/gossip.rs	91.32% <0.00%> (-0.63%)	⬇️
lightning/src/util/ser.rs	90.77% <0.00%> (-0.58%)	⬇️
lightning/src/ln/payment_tests.rs	98.73% <0.00%> (-0.53%)	⬇️
lightning/src/ln/msgs.rs	86.49% <0.00%> (-0.32%)	⬇️
lightning/src/ln/functional_tests.rs	96.80% <0.00%> (-0.31%)	⬇️
lightning-net-tokio/src/lib.rs	76.85% <0.00%> (-0.31%)	⬇️
... and 6 more

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