Offers without extra plugin

[thomash-acinq]

Enable basic offer management without the need for a plugin.

Offers allow a wide range of use-cases and it would be impossible to cover everything in eclair which is why we have relied on plugins to manage offers. However most users will not need advanced features, with this PR we aim to provide the basic features that will be enough for 95% of users. Advanced users can still use a plugin to manage more complex offers.

Supported use-cases:

• accepting donations
• selling items without inventory management
• compact offers using our public node id
• private offers where our identity is protected by a blinded path

[t-bast]

Mostly small architectural comments and nits at that point, the overall flow looks good!

[t-bast]

Looks mostly good to me, apart from the tests. I've done some refactoring and clean-up in #3030, but it should be quite straightforward (the biggest refactoring is DefaultOfferHandler, but it's purely refactoring).

[t-bast]

Some improvements to the tests, and we should be good to go!

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 85.24590% with 27 lines in your changes missing coverage. Please review.

Project coverage is 85.84%. Comparing base (10edb42) to head (867487f).
Report is 32 commits behind head on master.

Files with missing lines	Patch %	Lines
...r-core/src/main/scala/fr/acinq/eclair/Eclair.scala	0.00%	10 Missing ⚠️
.../main/scala/fr/acinq/eclair/db/DualDatabases.scala	25.00%	6 Missing ⚠️
...inq/eclair/payment/offer/DefaultOfferHandler.scala	84.84%	5 Missing ⚠️
...a/fr/acinq/eclair/payment/offer/OfferCreator.scala	86.66%	4 Missing ⚠️
.../main/scala/fr/acinq/eclair/db/pg/PgOffersDb.scala	97.29%	1 Missing ⚠️
...ala/fr/acinq/eclair/db/sqlite/SqliteOffersDb.scala	96.87%	1 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2976      +/-   ##
==========================================
- Coverage   86.02%   85.84%   -0.19%     
==========================================
  Files         228      234       +6     
  Lines       20452    21100     +648     
  Branches      846      846              
==========================================
+ Hits        17594    18113     +519     
- Misses       2858     2987     +129     

Files with missing lines	Coverage Δ
...air-core/src/main/scala/fr/acinq/eclair/Logs.scala	87.50% <100.00%> (+0.19%)	⬆️
...re/src/main/scala/fr/acinq/eclair/NodeParams.scala	92.14% <100.00%> (+0.14%)	⬆️
...ir-core/src/main/scala/fr/acinq/eclair/Setup.scala	74.11% <100.00%> (-0.89%)	⬇️
.../src/main/scala/fr/acinq/eclair/db/Databases.scala	84.40% <100.00%> (+0.29%)	⬆️
...e/src/main/scala/fr/acinq/eclair/db/OffersDb.scala	100.00% <100.00%> (ø)
...ain/scala/fr/acinq/eclair/db/pg/PgPaymentsDb.scala	98.61% <100.00%> (ø)
.../scala/fr/acinq/eclair/payment/Bolt12Invoice.scala	97.40% <100.00%> (-0.04%)	⬇️
...a/fr/acinq/eclair/payment/offer/OfferManager.scala	96.96% <100.00%> (+3.98%)	⬆️
...cinq/eclair/payment/receive/MultiPartHandler.scala	93.90% <100.00%> (+0.64%)	⬆️
.../scala/fr/acinq/eclair/payment/relay/Relayer.scala	90.00% <ø> (-0.20%)	⬇️
... and 8 more

... and 14 files with indirect coverage changes

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