Possibility to have just on-demand proposal fetching

[Waldz]

Needed for mobile

[codecov-io]

Codecov Report

Merging #1598 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master   #1598      +/-   ##
=========================================
- Coverage    47.8%   47.8%   -0.01%     
=========================================
  Files         289     289              
  Lines       11550   11519      -31     
=========================================
- Hits         5522    5507      -15     
+ Misses       5632    5618      -14     
+ Partials      396     394       -2

Impacted Files	Coverage Δ
session/pingpong/factory.go	0% <ø> (ø)	⬆️
session/pingpong/invoice_storage.go	93.47% <ø> (+18.85%)	⬆️
session/pingpong/exchange_message_tracker.go	90.21% <100%> (+1.1%)	⬆️
core/promise/methods/noop/dialog_mock.go	68.75% <0%> (-3.13%)	⬇️
core/connection/manager.go	83.22% <0%> (-0.65%)	⬇️
communication/nats/dialog/dialog_waiter.go	88.73% <0%> (+2.81%)	⬆️

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