validator: expose max active pubsub subscriptions to CLI (backport #18034)

[mergify]

This is an automatic backport of pull request #18034 done by Mergify.
Cherry-pick of 5efc48f has failed:

On branch mergify/bp/v1.6/pr-18034
Your branch is up to date with 'origin/v1.6'.

You are currently cherry-picking commit 5efc48fc6.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   core/src/rpc_pubsub.rs
	modified:   validator/src/main.rs

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   core/src/rpc_pubsub_service.rs

To fix up this pull request, you can check it out locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

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[mergify]

automerge label removed due to a CI failure

[codecov]

Codecov Report

Merging #18035 (4080f32) into v1.6 (1201ef1) will increase coverage by 0.0%.
The diff coverage is 100.0%.

@@           Coverage Diff           @@
##             v1.6   #18035   +/-   ##
=======================================
  Coverage    82.6%    82.6%           
=======================================
  Files         423      423           
  Lines      117091   117103   +12     
=======================================
+ Hits        96736    96748   +12     
  Misses      20355    20355