Proxy support through global agent bootstrap

[sius]

Description

Adds proxy support through global-agent:
If your action runs on a self-hosted GitHub Runner behind a proxy, you can enable proxy support through global-agent environment variables,
which supports the got request library (https://github.com/gajus/global-agent?tab=readme-ov-file#supported-libraries)

With the exception of the global-agent bootstrap, this solution is non-invasive and does not affect the implementation of the vault action;
it depends only on the integration between global-agent and the got request client.

An alternative implementation proposal with hpagent can be found here:

• add proxy support using hpagent #374

Relates OR Closes #0000

Checklist

• Added CHANGELOG entry (only for user-facing changes)

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[hashicorp-cla-app]

All committers have signed the CLA.

[sius]

The signing of the CLA can take place next week at the earliest