feat(toolkit-lib)!: generic messages do not have a code

[mrgrain]

In the original RFC we had introduced the concept of "standard" messages for which no backwards compatibility rules won't apply. In the first developer preview release of toolkit-lib this was implemented as message with a code ending in 0000 being consider "standard" (or "default") and other messages.

With the experience from the developer preview, we have found this way to handle the distinction slightly confusing and putting comparatively complicated work on the user (they need to know and check for the code ending in 0000). It was also easy to accidentally make a mistake by copy and pasting the message code and not noticing it's specialness.

Instead, we are now proposing to change this implementation: Messages and requests with a code will be subject to our backwards compatibility guarantees and can be relied upon by integrators. Standard messages will no longer have a code and must be treated purely informational. Note that all requests and any messages with a payload will always have a code. However some coded messages might not currently have a payload (but may in future according to our backwards compatibility rules).

This PR reflects these changes and updates the message registry with the respective documentation.

BREAKING CHANGE: The code field on messages is now optional. All messages with a code are subject to our backwards compatibility guarantees and can be relied upon by integrators. Messages that previously had a "default code" ending in 0000 no longer have a code and must be treated purely informational. Requests and messages with a payload are always coded.

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 28.57143% with 5 lines in your changes missing coverage. Please review.

Project coverage is 78.86%. Comparing base (0145866) to head (71ecf77).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
packages/aws-cdk/lib/cxapp/cloud-executable.ts	0.00%	2 Missing ⚠️
packages/aws-cdk/lib/cxapp/exec.ts	33.33%	2 Missing ⚠️
packages/aws-cdk/lib/cli/io-host/cli-io-host.ts	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #526      +/-   ##
==========================================
- Coverage   78.92%   78.86%   -0.06%     
==========================================
  Files          46       46              
  Lines        6980     6980              
  Branches      777      778       +1     
==========================================
- Hits         5509     5505       -4     
- Misses       1452     1456       +4     
  Partials       19       19              

Flag	Coverage Δ
suite.unit	78.86% <28.57%> (-0.06%)	⬇️

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