[OTAGENT-920] Set DD_OTELCOLLECTOR_INSTALLATION_METHOD on otel-agent container

[liustanley]

What does this PR do?

Injects DD_OTELCOLLECTOR_INSTALLATION_METHOD=kubernetes into the otel-agent container when the Operator reconciles the node agent pod spec. This env var maps to the otelcollector.installation_method core config key, which the converter reads and injects into the extensions.datadog config so the Datadog extension can report the installation method in collector metadata payloads.

Motivation

Part of OTAGENT-920. Intended for the 7.78.0 agent release.

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Codecov Report

❌ Patch coverage is 50.00000% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 40.06%. Comparing base (395974a) to head (347f4a0).

Files with missing lines	Patch %	Lines
...controller/datadogagent/global/otelagentgateway.go	0.00%	4 Missing ⚠️

❌ Your patch status has failed because the patch coverage (50.00%) is below the target coverage (80.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2846   +/-   ##
=======================================
  Coverage   40.05%   40.06%           
=======================================
  Files         319      319           
  Lines       28031    28039    +8     
=======================================
+ Hits        11229    11233    +4     
- Misses      15979    15983    +4     
  Partials      823      823           

Flag	Coverage Δ
unittests	40.06% <50.00%> (+<0.01%)	⬆️

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

Files with missing lines	Coverage Δ
...ller/datadogagent/feature/otelcollector/feature.go	77.13% <100.00%> (+0.41%)	⬆️
...controller/datadogagent/global/otelagentgateway.go	0.00% <0.00%> (ø)

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