refactor!: rename Handler to MessageHandler

[TimoGlastra]

Rename Handler to MessageHandler based on feedback from PR #1158

BREAKING CHANGE: Handler has been renamed to MessageHandler to be more descriptive, along with related types and methods. This means:

• Handler is now MessageHandler
• HandlerInboundMessage is now MessageHandlerInboundMessage
• Dispatcher.registerHandler is now Dispatcher.registerMessageHandlers

[codecov-commenter]

Codecov Report

Merging #1161 (9502072) into main (0e89e6c) will not change coverage.
The diff coverage is 96.99%.

@@           Coverage Diff           @@
##             main    #1161   +/-   ##
=======================================
  Coverage   88.77%   88.77%           
=======================================
  Files         706      706           
  Lines       16503    16503           
  Branches     2678     2678           
=======================================
  Hits        14651    14651           
  Misses       1845     1845           
  Partials        7        7           

Impacted Files	Coverage Δ
packages/core/src/modules/proofs/ProofService.ts	68.96% <ø> (ø)
.../modules/connections/handlers/AckMessageHandler.ts	71.42% <50.00%> (ø)
...ctions/handlers/TrustPingResponseMessageHandler.ts	71.42% <50.00%> (ø)
...cation/handlers/V1RevocationNotificationHandler.ts	71.42% <50.00%> (ø)
...cation/handlers/V2RevocationNotificationHandler.ts	71.42% <50.00%> (ø)
...ol/v1/handlers/V1CredentialProblemReportHandler.ts	71.42% <50.00%> (ø)
...ol/v2/handlers/V2CredentialProblemReportHandler.ts	71.42% <50.00%> (ø)
...c/modules/routing/handlers/MediationDenyHandler.ts	62.50% <50.00%> (ø)
packages/action-menu/src/ActionMenuApi.ts	97.82% <100.00%> (ø)
...enu/src/handlers/ActionMenuProblemReportHandler.ts	100.00% <100.00%> (ø)
... and 71 more

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