Rpc: Add getStakeActivation endpoint (bp #10902)

[mergify]

This is an automated backport of pull request #10902 done by Mergify

Cherry-pick of 4de0713 has failed:

On branch mergify/bp/v1.1/pr-10902
Your branch is up to date with 'origin/v1.1'.

You are currently cherry-picking commit 4de0713aa.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:

	modified:   client/src/rpc_config.rs
	modified:   client/src/rpc_response.rs

Unmerged paths:
  (use "git add <file>..." to mark resolution)

	both modified:   core/src/rpc.rs
	both modified:   docs/src/apps/jsonrpc-api.md

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[mergify]

automerge label removed due to a CI failure

[codecov]

Codecov Report

Merging #11185 into v1.1 will increase coverage by 0.0%.
The diff coverage is 1.9%.

@@          Coverage Diff           @@
##            v1.1   #11185   +/-   ##
======================================
  Coverage   81.1%    81.1%           
======================================
  Files        292      292           
  Lines      68947    68997   +50     
======================================
+ Hits       55965    56008   +43     
- Misses     12982    12989    +7