* implement SSRF protection header
* add test for SSRF protection header
* cleanup
* refactor
* implement SSRF header on a per-listener basis
* cleanup
* cleanup
* creat unit test for agent SSRF
* improve unit test for agent SSRF
* add VaultRequest SSRF header to CLI
* fix unit test
* cleanup
* improve test suite
* simplify check for Vault-Request header
* add constant for Vault-Request header
* improve test suite
* change 'config' to 'agentConfig'
* Revert "change 'config' to 'agentConfig'"
This reverts commit 14ee72d21f.
* do not remove header from request
* change header name to X-Vault-Request
* simplify http.Handler logic
* cleanup
* simplify http.Handler logic
* use stdlib errors package
