Support registered evaluations in `lf.eval`.

[copybara-service[bot]]

Support registered evaluations in lf.eval.

• Add lf.eval.register to enable named evaluations.
• Add lf.eval.patch_xxx to support experiment patching.
• Add lf.eval.get to obtain evaluations with patching.
• Remove app_run from lf.eval framework.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 84.00000% with 12 lines in your changes are missing coverage. Please review.

Project coverage is 95.06%. Comparing base (ae48c26) to head (c58cd12). Report is 1 commits behind head on main.

Files	Patch %	Lines
langfun/core/eval/base.py	82.60%	5 Missing and 7 partials :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #198 +/- ## ========================================== - Coverage 95.10% 95.06% -0.05% ========================================== Files 58 58 Lines 5130 5187 +57 Branches 897 908 +11 ========================================== + Hits 4879 4931 +52 - Misses 128 130 +2 - Partials 123 126 +3 ```

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