Add SYSTEM commands and Keeper 4LW for jemalloc

[antonio2368]

Changelog category (leave one):

• Improvement

Changelog entry (a user-readable short description of the changes that goes to CHANGELOG.md):

Add SYSTEM JEMALLOC PURGE for purging unused jemalloc pages, SYSTEM JEMALLOC [ ENABLE | DISABLE | FLUSH ] PROFILE for controlling jemalloc profile if the profiler is enabled.
Add jemalloc-related 4LW command in Keeper: jmst for dumping jemalloc stats, jmfp, jmep, jmdp for controlling jemalloc profile if the profiler is enabled.

cc @KochetovNicolai

Documentation entry for user-facing changes

• Documentation is written (mandatory for new features)

Information about CI checks: https://clickhouse.com/docs/en/development/continuous-integration/

[robot-ch-test-poll4]

This is an automated comment for commit d104072 with description of existing statuses. It's updated for the latest CI running

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Check name	Description	Status
Performance Comparison	Measure changes in query performance. The performance test report is described in detail here. In square brackets are the optional part/total tests	❌ failure
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[antonio2368]

Stress tests seem to be broken but unrelated to this PR