Enable JVM proxy system properties in Key Vault JCA HTTP client

[waiet]

Problem

Fixes #28801.

The Key Vault JCA provider uses an internal Apache HttpClient in HttpUtil instead of the standard Azure SDK HTTP pipeline. As a result, standard JVM proxy properties such as https.proxyHost, https.proxyPort, http.proxyHost, http.proxyPort, and http.nonProxyHosts were not honored when the provider was used behind a corporate proxy.

Solution

Configure the internal Apache HttpClient builder with useSystemProperties() while preserving the existing SSL/truststore connection manager behavior.

This PR also adds unit-level proxy coverage and documents a jarsigner proxy example using standard JVM system properties.

Testing

Added a unit test that sets http.proxyHost and http.proxyPort, serves a local proxy response, and verifies HttpUtil.get routes through the proxy. The test locks JVM system properties while it runs to avoid interference with parallel JUnit execution.

Validated with module-level tests:
mvn -f sdk/keyvault/azure-security-keyvault-jca/pom.xml -DskipITs -Dgpg.skip -Dspotbugs.skip -Drevapi.skip -Dspotless.skip=true -Dcodesnippet.skip=true -Djacoco.skip=true -DheapDumpOnOom= test

Result: 80 tests, 0 failures, 0 errors, 29 skipped.

The root mvn -pl sdk/keyvault/azure-security-keyvault-jca -DskipITs -Dgpg.skip -Dspotbugs.skip -Drevapi.skip test command could not run in this sparse checkout because the root POM references modules not present locally.

[github-actions]

Thank you for your contribution @waiet! We will review the pull request and get back to you soon.

[waiet]

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