[fix][broker] Check migration cluster exist when enable namespace migration in blue-green migration

[hanmz]

Motivation

#21367 Support to migrate topics from blue to green cluster per namespace

But when we enable namespace migration, there was no check if the migrate cluster existed. There will be some unusual situations here.

Minimal reproduce step:
1.Start a single broker node and create a tenant test-tenant
sh bin/pulsar-admin tenants create test-tenant

2.Create a new namespace and a topic.
sh bin/pulsar-admin namespaces create test-tenant/test-ns
sh bin/pulsar-admin topics create test-tenant/test-ns/test-topic

3.Enable namespaces migration
sh bin/pulsar-admin namespaces update-migration-state --migrated test-tenant/test-ns

We will see the following log:

Modifications

1. Check migration cluster exist when enable namespace migration.
2. Determine whether clusterData is exists and isNamespaceMigrationEnabled=true satisfied at the same time in method getMigratedClusterUrlAsync

Documentation

• doc
• doc-required
• doc-not-needed
• doc-complete

[hanmz]

Please help me review it when you have time. @vraulji567 @rdhabalia @eolivelli Thank you so much.

[hanmz]

/pulsarbot rerun-failure-checks

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 83.33333% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 72.90%. Comparing base (bbc6224) to head (4bd2aa4).
Report is 111 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #22362      +/-   ##
============================================
- Coverage     73.57%   72.90%   -0.68%     
+ Complexity    32624    32304     -320     
============================================
  Files          1877     1892      +15     
  Lines        139502   145784    +6282     
  Branches      15299    16776    +1477     
============================================
+ Hits         102638   106279    +3641     
- Misses        28908    31227    +2319     
- Partials       7956     8278     +322     

Flag	Coverage Δ
inttests	27.94% <0.00%> (+3.36%)	⬆️
systests	25.30% <0.00%> (+0.97%)	⬆️
unittests	72.04% <83.33%> (-0.81%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
...rg/apache/pulsar/broker/service/AbstractTopic.java	87.98% <100.00%> (ø)
...pache/pulsar/broker/admin/impl/NamespacesBase.java	74.83% <80.00%> (+1.75%)	⬆️

... and 175 files with indirect coverage changes