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]

@microsoft-github-policy-service agree company="IThink s. r. o."

[waiet]

Hi @moarychan , this PR adds support for standard JVM proxy system properties in the azure-security-keyvault-jca internal Apache HttpClient, The PR is ready for review when you have a chance. Thank you!

[joachim-johansson]

This change would be very helpful also for us. This is exactly what we need. We have tried the same change and had it our test environments for while now. Works well.

[waiet]

This change would be very helpful also for us. This is exactly what we need. We have tried the same change and had it our test environments for while now. Works well.

It would be great, if this feature will be released in next version. But the PR is still waiting for review :-(