Use system proxy settings in REST and GRAPHQL plugins

[ludomikula]

Proposed changes

Adds possibility to use system proxy if configured for REST and GraphQL plugins.
Introduces usage of proxy username/password.

System environment properties recognized:

• http.proxyHost
• http.proxyPort
• http.proxyUsername
• http.proxyPassword

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• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Update (if none of the other choices apply)

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Further comments

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[FalkWolsky]

A truly helpful contribution!! Thank you very much. This will help to increase network security and flexibility a lot!