[FT] Provide an interface for easier edit of parametrizable metrics

[clefourrier]

Issue encountered

When using a metric which is parametrizable, you need to create a new custom metric to edit default parameters - for some metrics like the BertScorer it's harder to do, see #310 point 3

Solution/Feature

We need to provide an interface or examples on this

[chuandudx]

Thank you! Dropping in related ideas here as they come up:
Are we open to passing in bert_scorer into BertScore initialization?
reference:

lighteval/src/lighteval/metrics/metrics_sample.py

Line 539 in 7295c78

class BertScore: