Add optional external_id field to SCIM users and groups

[alexott]

External ID could be propagated from linked identity provider, and is used for things like Unity Catalog

[codecov]

Codecov Report

Merging #927 (6ef27b3) into master (57d334d) will increase coverage by 0.05%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #927      +/-   ##
==========================================
+ Coverage   83.56%   83.62%   +0.05%     
==========================================
  Files          93       93              
  Lines        8648     8677      +29     
==========================================
+ Hits         7227     7256      +29     
  Misses        910      910              
  Partials      511      511              

Impacted Files	Coverage Δ
identity/scim.go	92.59% <ø> (ø)
identity/data_current_user.go	94.73% <100.00%> (+0.79%)	⬆️
identity/data_group.go	82.60% <100.00%> (+0.79%)	⬆️
identity/data_user.go	96.61% <100.00%> (+0.69%)	⬆️
identity/groups.go	88.09% <100.00%> (+0.29%)	⬆️
identity/resource_group.go	100.00% <100.00%> (ø)
identity/resource_user.go	87.75% <100.00%> (+0.79%)	⬆️

[nfx]

please also add an entry to changelog