chore(slack): track timeout errors from sdk WebClient

[cathteng]

These are already very occasionally being emitted as Sentry errors, but it would be good to look at them relative to all Slack responses being recorded in DD. This error is emitted from Slack, and is not due to us.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 78.02%. Comparing base (280454e) to head (06ba4ad). Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #73546      +/-   ##
==========================================
+ Coverage   77.01%   78.02%   +1.00%     
==========================================
  Files        6638     6638              
  Lines      296878   296880       +2     
  Branches    51122    51123       +1     
==========================================
+ Hits       228655   231639    +2984     
+ Misses      61817    58967    -2850     
+ Partials     6406     6274     -132     

Files	Coverage Δ
src/sentry/integrations/slack/sdk_client.py	77.63% <100.00%> (+0.60%)	:arrow_up:

... and 272 files with indirect coverage changes