Define a default MessageRouter implementation

[jkczyz]

Otherwise, users need to define one when using "Simple" type aliases.

[TheBlueMatt]

Basically LGTM once CI passes.

[codecov-commenter]

Codecov Report

Patch and project coverage have no change.

Comparison is base (bd12067) 90.32% compared to head (129dd30) 90.32%.

❗ Your organization is not using the GitHub App Integration. As a result you may experience degraded service beginning May 15th. Please install the Github App Integration for your organization. Read more.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2384   +/-   ##
=======================================
  Coverage   90.32%   90.32%           
=======================================
  Files         106      106           
  Lines       54951    54996   +45     
  Branches    54951    54996   +45     
=======================================
+ Hits        49633    49674   +41     
- Misses       5318     5322    +4     

Impacted Files	Coverage Δ
lightning-background-processor/src/lib.rs	82.58% <ø> (ø)
lightning/src/ln/peer_handler.rs	61.77% <ø> (ø)
lightning/src/onion_message/messenger.rs	86.19% <0.00%> (-1.25%)	⬇️

... and 2 files with indirect coverage changes

☔ View full report in Codecov by Sentry.
📢 Do you have feedback about the report comment? Let us know in this issue.