store: add custom_config field

[milinddethe15]

• I added CHANGELOG entry for this change.
• Change is not relevant to the end user.

Changes

New custom_config field to set hedging config in bucket.yml

Verification

[yeya24]

I am a bit confused about this PR.

https://github.com/thanos-io/thanos/pull/7860/files#diff-3750e2beab2a47b781819ff1d3ecf9eeea52b3a6a10612de00cd13aa94f318ecR585
we already documented how to configure hedged reuqest in the previous PR. You meant the configuration was actually not working in that PR, and this PR fixes it?

[milinddethe15]

It just improves the docs and structure for yaml.

• config can be set as following:

custom_config:
  hedging_config:
    enabled: false
    up_to: 0
    quantile: 0

• Now it is included in the example bucket.yaml in the docs.

custom_config can be extra field, which can be removed but adding hedging_config in the example bucket.yaml in docs was my main motive.