[DocDB] yb-admin command to clear the block cache

Jira Link: DB-14995

Description

Provide a yb-admin command to clear the block cache. This is useful in performance experiments where we would want to clean up the block cache after one experiment, so as to not allow the block cache to interfere with the results of the next experiment.

This can be implemented the same way as already done for flush_table yb-admin command but for simplicity of use and implementation just clear the whole block cache on all tservers and don't take any parameters.

Issue Type

kind/enhancement

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