ENH improve benchmark metadata handeling

[tomMoral]

On the result website, objective.py is not available to collect the benchmark metadata.
These metadata are stored in benchmark_meta.json which is created when using benchopt publish.

As only pretty_name is available for now, the reading of these data was manually done.
however, this is not very convenient in generate_html which handles both cases, when objective.py or benchmark_meta.json are available.
I propose to store all the metadata required by generate_html directly in the benchmark class and automatically retrieve them when objective.py is not available but benchmark_meta.json is.

[codecov-commenter]

Codecov Report

Merging #621 (d7818c4) into main (b7ec43e) will increase coverage by 0.15%.
The diff coverage is 15.25%.

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@@            Coverage Diff             @@
##             main     #621      +/-   ##
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+ Coverage   55.98%   56.14%   +0.15%     
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  Files          45       45              
  Lines        2890     2882       -8     
  Branches      528      530       +2     
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  Hits         1618     1618              
+ Misses       1154     1144      -10     
- Partials      118      120       +2