Add paused_purpose to INFO CLIENTS

[enjoy-binbin]

In #1519, we added paused_actions and paused_timeout_milliseconds,
it would be helpful if we add the paused_purpose since users also
want to know the purpose for the pause.

Currently available options:

• client_command: trigger by CLIENT PAUSE command.
• during_shutdown: during shutdown, primary waits the replicas to catch up the offset.
• during_failover: during failover, primary waits the replica to catch up the offset.
• none

[codecov]

Codecov Report

Attention: Patch coverage is 80.00000% with 4 lines in your changes missing coverage. Please review.

Project coverage is 70.95%. Comparing base (2a1a65b) to head (376fd33).
Report is 1 commits behind head on unstable.

Files with missing lines	Patch %	Lines
src/networking.c	71.42%	4 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##           unstable    #1564      +/-   ##
============================================
+ Coverage     70.78%   70.95%   +0.16%     
============================================
  Files           120      120              
  Lines         65046    65077      +31     
============================================
+ Hits          46045    46176     +131     
+ Misses        19001    18901     -100     

Files with missing lines	Coverage Δ
src/server.c	87.66% <100.00%> (+0.04%)	⬆️
src/server.h	100.00% <ø> (ø)
src/networking.c	88.40% <71.42%> (+0.04%)	⬆️

... and 14 files with indirect coverage changes

[hwware]

I suggest removing the String from function name getPausedPurposeString(), char* already represents the return type. I know in server.h, some function names keep the String even the return type is char*, and some do not. Let us keep simple and normalize it.