Add `log_key_prefix` parameter to `LearningRateMonitor` (#21612)

* Add `log_key_prefix` parameter to `LearningRateMonitor` callback

Allow users to prepend a configurable prefix to all metric names logged
by `LearningRateMonitor`. This is useful for grouping learning rate
metrics in loggers like TensorBoard (e.g., `optim/lr-Adam` instead of
`lr-Adam`).

Fixes #21590

* Update src/lightning/pytorch/CHANGELOG.md

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Co-authored-by: Deependu <deependujha21@gmail.com>