Introduce `lf.generate_class`.

[copybara-service[bot]]

Introduce lf.generate_class.

Usage:

lm = lf.llms.GeminiPro()
cls = lf.generate_class('Person', 'The basic description of a celebrity.', lm=lm)
lf.query('Who is Larry Page', cls, lm=lm)

This CL also introduces:

• lf.source_form to get the source code form representation of a class (and its dependencies) and a symbolic object.
• refactoring the default examples of lf.parse and lf.describe from constant objects into functions, which could disclose the class definitions of the examples from external access.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 90.90909% with 9 lines in your changes are missing coverage. Please review.

Project coverage is 95.96%. Comparing base (b316e8c) to head (9a809ab).

Files	Patch %	Lines
langfun/core/structured/schema_generation.py	82.60%	4 Missing and 4 partials :warning:
langfun/core/structured/schema.py	95.83%	0 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #154 +/- ## ========================================== - Coverage 96.08% 95.96% -0.13% ========================================== Files 54 55 +1 Lines 4371 4434 +63 Branches 948 775 -173 ========================================== + Hits 4200 4255 +55 - Misses 86 90 +4 - Partials 85 89 +4 ```

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