feat(sdk-core): Add User-Agent header with ft/warmup marker to HTTP client warm-up calls

[joviegas]

Motivation and Context

This change adds a User-Agent header to warm-up requests: the system user agent (SDK
version, Java version, OS, JVM metadata) plus an ft/warmup feature marker. It reuses
existing user-agent construction (SdkUserAgentBuilder, UserAgentConstant)

Modifications

• HttpClientWarmer: add constants HEADER_USER_AGENT ("User-Agent") and
  WARM_UP_FEATURE_ID ("warmup"), and a static warmUpUserAgent() helper. The helper
  builds the system user agent with
  SdkUserAgentBuilder.buildSystemUserAgentString(SystemUserAgent.getOrCreate()) and appends
  ft/warmup with UserAgentConstant.appendSpaceAndField.
• SyncHttpClientWarmer: build the value once in warmAll() and set it on the warm-up
  SdkHttpRequest.
• AsyncHttpClientWarmer: build the value once in warmAll() and set it on the warm-up
  SdkHttpFullRequest.

Testing

• Enabled org.apache.http.wire debug logging and ran the warm-up. The request carried the
  header:

16:20:59.004 [main] DEBUG org.apache.http.wire - http-outgoing-0 >> "User-Agent: aws-sdk-java/2.51.4-SNAPSHOT os/Mac_OS_X#56.7.8 lang/java#19.0.20.1 md/OpenJDK_64-Bit_Server_VM#11.0.20.1+9-LTS md/vendor#Some.com_Inc. md/en_US ft/warmup[\r][\n]"

License

• I confirm that this pull request can be released under the Apache 2 license

[github-actions]

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