sys/config: config state endpoint (#7424)

* sys/config: initial work on adding config state endpoint

* server/config: add tests, fix Sanitized method

* thread config through NewTestCluster's config to avoid panic on dev modes

* properly guard endpoint against request forwarding

* add http tests, guard against panics on nil RawConfig

* ensure non-nil rawConfig on NewTestCluster cores

* update non-forwarding logic

* fix imports; use no-forward handler

* add missing config test fixture; update gitignore

* return sanitized config as a map

* fix test, use deep.Equal to check for equality

* fix http test

* minor comment fix

* config: change Sanitized to return snake-cased keys, update tests

* core: hold rlock when reading config; add docstring

* update docstring

.gitignore

@@ -48,6 +48,7 @@ Vagrantfile
 # Configs
 *.hcl
 !command/agent/config/test-fixtures/*.hcl
+!command/server/test-fixtures/*.hcl
 
 
 .DS_Store