feat: add detach subscription rpc

[yoshi-automation]

This PR was generated using Autosynth. 🌈

Synth log will be available here:
https://source.cloud.google.com/results/invocations/3f2d59a9-201f-4a67-b247-7356a3ac82ae/targets

PiperOrigin-RevId: 313276022
Source-Link: googleapis/googleapis@f5f268f

[codecov]

Codecov Report

Merging #216 into master will increase coverage by 0.12%.
The diff coverage is 89.18%.

@@             Coverage Diff              @@
##             master     #216      +/-   ##
============================================
+ Coverage     79.05%   79.17%   +0.12%     
- Complexity      312      316       +4     
============================================
  Files            21       21              
  Lines          2841     2877      +36     
  Branches        153      153              
============================================
+ Hits           2246     2278      +32     
- Misses          533      537       +4     
  Partials         62       62              

Impacted Files	Coverage Δ	Complexity Δ
...com/google/cloud/pubsub/v1/TopicAdminSettings.java	19.60% <0.00%> (-0.81%)	5.00 <0.00> (ø)
...com/google/cloud/pubsub/v1/stub/PublisherStub.java	6.25% <0.00%> (-0.42%)	1.00 <0.00> (ø)
...le/cloud/pubsub/v1/stub/PublisherStubSettings.java	83.56% <90.90%> (+0.29%)	21.00 <1.00> (+1.00)
...a/com/google/cloud/pubsub/v1/TopicAdminClient.java	58.95% <100.00%> (+0.48%)	34.00 <2.00> (+2.00)
...google/cloud/pubsub/v1/stub/GrpcPublisherStub.java	96.09% <100.00%> (+0.33%)	19.00 <1.00> (+1.00)

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[anguillanneuf]

I only had to resolve a merge conflict in synth.metadata.