fix: allow agent without inbound endpoint to connect when using multi-use invitation

[TimoGlastra]

Quite an edge case, but when an agent without an inbound endpoint (e.g. mobile wallet connecting to mediator) would respond to a multi-use connection invitation (quite common with mediators) the session would be stored for the multi-use invitation connection. But during the processing of the request a new connection will be created, which means the session won't be reused afterwards because it's tied to the multi-use invitation connection.

Normally this isn't that big a deal, as the next time a message is sent using the new connection the session can be reused, but for agent without an inbound transport this made it impossible to connect to AFJ.

This PR adds the sessionId to the inbound message, and later attaches it to the outbound message. This is only used when directly responding to an incoming message. Wasn't sure if attaching the sessionId is the best here, but couldn't think of other straightforward approaches. We basically want to attach it to the inbound message, so we can just respond to the inbound session (instead of finding a session by the connection id, which is still used for a lot of other use cases)

Also changed the subject transport a bit to not require an inbound transport anymore when no inbound endpoint is configured (better reflects real-world setup)

Fixes #483

[codecov-commenter]

Codecov Report

Merging #712 (11feb8b) into main (75fb99e) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #712      +/-   ##
==========================================
+ Coverage   86.81%   86.84%   +0.02%     
==========================================
  Files         358      358              
  Lines        7806     7812       +6     
  Branches     1205     1316     +111     
==========================================
+ Hits         6777     6784       +7     
+ Misses       1028      974      -54     
- Partials        1       54      +53     

Impacted Files	Coverage Δ
packages/core/src/types.ts	100.00% <ø> (ø)
packages/core/src/agent/Dispatcher.ts	83.63% <100.00%> (+0.30%)	⬆️
packages/core/src/agent/MessageReceiver.ts	77.68% <100.00%> (+0.18%)	⬆️
packages/core/src/agent/MessageSender.ts	74.26% <100.00%> (+0.58%)	⬆️
packages/core/src/agent/TransportService.ts	100.00% <100.00%> (+7.40%)	⬆️
...ges/core/src/agent/models/InboundMessageContext.ts	92.30% <100.00%> (+0.64%)	⬆️
packages/core/src/storage/IndyStorageService.ts	89.32% <0.00%> (-0.98%)	⬇️
packages/node/src/NodeFileSystem.ts	37.14% <0.00%> (ø)
packages/core/src/crypto/JwsService.ts	90.90% <0.00%> (ø)
packages/core/src/utils/transformers.ts	69.44% <0.00%> (ø)
... and 27 more

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[berendsliedrecht]

Not too much noticeable for me. I am not that well familiar with the inbound/outbound transport so I just commented more on some general code quality.

[berendsliedrecht]

This can be done nicely in a ternary expression instead of a let with if/else

[TimoGlastra]

I tried that, but that won't work in this case because I want to find as session by id if the session id is present, and if there is no session form that first step then I want to find the session by connection id.

So it could be that both methods are called, and with a ternerary only one of the two will be called