add user-client mapping.

[Shuen14]

As you can see in https://github.com/redis/redis/pull/12245 ,user-client mapping has been added to the server to facilitate statistics on the usage of each user connection.

client list user return specified user connection info

127.0.0.1:6379> CLIENT list user doug
id=5 addr=127.0.0.1:59469 laddr=127.0.0.1:6379 fd=9 name= age=33 idle=28 flags=N db=0 sub=0 psub=0 ssub=0 multi=-1 watch=0 qbuf=0 qbuf-free=0 argv-mem=0 multi-mem=0 rbs=1024 rbp=0 obl=0 oll=0 omem=0 tot-mem=1824 events=r cmd=auth user=doug redir=-1 resp=2 lib-name= lib-ver=
id=6 addr=127.0.0.1:59490 laddr=127.0.0.1:6379 fd=10 name= age=19 idle=15 flags=N db=0 sub=0 psub=0 ssub=0 multi=-1 watch=0 qbuf=0 qbuf-free=0 argv-mem=0 multi-mem=0 rbs=1024 rbp=0 obl=0 oll=0 omem=0 tot-mem=1824 events=r cmd=auth user=doug redir=-1 resp=2 lib-name= lib-ver=

client count return specified user connetcion count

127.0.0.1:6379> CLIENT COUNT doug
"2"

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 50.63291% with 39 lines in your changes are missing coverage. Please review.

Project coverage is 68.50%. Comparing base (93f8a19) to head (689db89). Report is 8 commits behind head on unstable.

Additional details and impacted files

@@             Coverage Diff              @@
##           unstable     #405      +/-   ##
============================================
+ Coverage     68.43%   68.50%   +0.06%     
============================================
  Files           109      109              
  Lines         61681    61760      +79     
============================================
+ Hits          42214    42309      +95     
+ Misses        19467    19451      -16     

Files	Coverage Δ
src/acl.c	89.02% <100.00%> (+0.04%)	:arrow_up:
src/commands.def	100.00% <ø> (ø)
src/networking.c	83.84% <46.57%> (-1.24%)	:arrow_down:

... and 10 files with indirect coverage changes