Add config for User-Agent suffix

[nojnhuh]

Closes #1827

What this PR does / why we need it: This PR adds an AZURE_USER_AGENT_SUFFIX configuration option to allow appending to the default User-Agent used for Azure HTTP clients.

Special notes for your reviewer:

Getting this wired up felt a little clunky, so any feedback on how to smooth that out is welcome.

I also wasn't sure exactly how best to add tests for this config, so any advice there would be appreciated. And tips on exactly where to look with ASO fully up and running to see if the user agent is really being set as expected.

How does this PR make you feel:

If applicable:

• this PR contains documentation
• this PR contains tests

[codecov-commenter]

Codecov Report

Attention: 1075 lines in your changes are missing coverage. Please review.

Comparison is base (f6ab7ee) 53.08% compared to head (99313f5) 53.43%.
Report is 26 commits behind head on main.

Files	Patch %	Lines
...1/storage/managed_clusters_agent_pool_types_gen.go	70.23%	216 Missing and 140 partials ⚠️
...230202preview/storage/managed_cluster_types_gen.go	58.30%	148 Missing and 108 partials ⚠️
...w/storage/managed_clusters_agent_pool_types_gen.go	53.74%	86 Missing and 50 partials ⚠️
...1/storage/compat/service_mesh_profile_types_gen.go	50.99%	60 Missing and 39 partials ⚠️
...ge/compat/service_mesh_profile_status_types_gen.go	52.97%	56 Missing and 39 partials ⚠️
...ompat/cluster_upgrade_settings_status_types_gen.go	55.55%	24 Missing and 16 partials ⚠️
...orage/compat/cluster_upgrade_settings_types_gen.go	62.22%	20 Missing and 14 partials ⚠️
...rofile_vertical_pod_autoscaler_status_types_gen.go	74.13%	9 Missing and 6 partials ⚠️
...caler_profile_vertical_pod_autoscaler_types_gen.go	74.13%	9 Missing and 6 partials ⚠️
...v1api20210501/storage/managed_cluster_types_gen.go	50.00%	4 Missing and 4 partials ⚠️
... and 4 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3650      +/-   ##
==========================================
+ Coverage   53.08%   53.43%   +0.34%     
==========================================
  Files        1286     1356      +70     
  Lines      406699   460920   +54221     
==========================================
+ Hits       215894   246289   +30395     
- Misses     159804   177613   +17809     
- Partials    31001    37018    +6017     

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[theunrepentantgeek]

Generally looks good, but I have some ideas about an alternative way to plumb the value in.

[theunrepentantgeek]

This will only be used if the code that calls NewGenericClient() doesn't specify a user agent in the GenericClientOptions struct passed in. I don't think that's ideal, as the configured setting wouldn't be included.

I haven't checked where GenericClientIOptions.UserAgent is set, but I'm wondering if the value used there should be used as a part of the UA, not the whole thing.

[theunrepentantgeek]

/ok-to-test sha=fc7fcd4

[theunrepentantgeek]

One minor comment, otherwise looks good. I'll trigger testing and merge once that's addressed.

[theunrepentantgeek]

/ok-to-test sha=99313f5

[theunrepentantgeek]

/azp run

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