Fix the assignment of a delivery service to a server, to respect capabilities

[rimashah25]

This PR fixes the issue of attempting to execute direct DS assignment in TP

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Which Traffic Control components are affected by this PR?

• Traffic Ops
• Traffic Portal

What is the best way to verify this PR?

Run Traffic Ops and Traffic Portal.
Create a deliveryservice and assign a topology to it.
Now, try to assign a server independently to this DS.
Make sure that it works.

Now try the above operations with and without required capabilities on the DS, and server capabilities on the server.

If this is a bugfix, which Traffic Control versions contained the bug?

• master

PR submission checklist

• This PR has tests
• This PR has documentation
• This PR has a CHANGELOG.md entry
• This PR DOES NOT FIX A SERIOUS SECURITY VULNERABILITY (see the Apache Software Foundation's security guidelines for details)

[codecov]

Codecov Report

Attention: 15 lines in your changes are missing coverage. Please review.

Comparison is base (fa231d7) 65.64% compared to head (adbdd1e) 31.89%.

Files	Patch %	Lines
...ffic_ops_golang/deliveryservice/servers/servers.go	0.00%	13 Missing ⚠️
lib/go-tc/deliveryservice_servers.go	0.00%	1 Missing ⚠️
...ps/traffic_ops_golang/server/servers_assignment.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##             master    #7878       +/-   ##
=============================================
- Coverage     65.64%   31.89%   -33.75%     
  Complexity       98       98               
=============================================
  Files           323      719      +396     
  Lines         12836    82838    +70002     
  Branches        970      970               
=============================================
+ Hits           8426    26424    +17998     
- Misses         4050    54252    +50202     
- Partials        360     2162     +1802     

Flag	Coverage Δ
golib_unit	53.84% <0.00%> (?)
grove_unit	12.02% <ø> (?)
t3c_unit	5.88% <ø> (?)
traffic_monitor_unit	26.44% <ø> (?)
traffic_ops_unit	21.67% <0.00%> (?)
traffic_portal_v2	74.30% <ø> (-0.06%)	⬇️
traffic_stats_unit	10.78% <ø> (?)
unit_tests	29.22% <0.00%> (-45.13%)	⬇️

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