maint(authd): Expose public API to build and parse UI layouts

[3v1n0]

This is the continuation of https://github.com/ubuntu/authd/pull/626 that allows to have a single codebase to both parse and generate UI layouts from the client and broker sides.

Compared to the initial version of this change, now the types that we export are still based on the proto definitions, but we generate them with a new code generator that ensures that types can be exported and that does 1:1 copies when required.

Broker side: https://github.com/ubuntu/authd-oidc-brokers/pull/234

UDENG-5522

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 90.63232% with 40 lines in your changes missing coverage. Please review.

Project coverage is 83.13%. Comparing base (36511cd) to head (ee0d2d8). Report is 366 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/examplebroker/broker.go	75.00%	12 Missing and 7 partials :warning:
internal/services/pam/pam.go	66.66%	4 Missing and 2 partials :warning:
internal/proto/authd/authmode.go	50.00%	5 Missing :warning:
brokers/auth/authmode.go	0.00%	4 Missing :warning:
brokers/auth/mode.go	93.33%	2 Missing and 1 partial :warning:
brokers/layouts/uilayout.go	91.89%	2 Missing and 1 partial :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #788      +/-   ##
==========================================
- Coverage   83.43%   83.13%   -0.31%     
==========================================
  Files          83      101      +18     
  Lines        8689     9880    +1191     
  Branches       74       74              
==========================================
+ Hits         7250     8214     +964     
- Misses       1111     1277     +166     
- Partials      328      389      +61     

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