authenticate: make service http only

- Rename SessionState to State to avoid stutter.
- Simplified option validation to use a wrapper function for base64 secrets.
- Removed authenticates grpc code.
- Abstracted logic to load and validate a user's authenticate session.
- Removed instances of url.Parse in favor of urlutil's version.
- proxy: replaces grpc refresh logic with forced deadline advancement.
- internal/sessions: remove rest store; parse authorize header as part of session store.
- proxy: refactor request signer
- sessions: remove extend deadline (fixes #294)
- remove AuthenticateInternalAddr
- remove AuthenticateInternalAddrString
- omit type tag.Key from declaration of vars TagKey* it will be inferred
  from the right-hand side
- remove compatibility package xerrors
- use cloned http.DefaultTransport as base transport

internal/telemetry/metrics/const.go

@@ -8,12 +8,12 @@ import (
 
 // The following tags are applied to stats recorded by this package.
 var (
-	TagKeyHTTPMethod  tag.Key = tag.MustNewKey("http_method")
-	TagKeyService     tag.Key = tag.MustNewKey("service")
-	TagKeyGRPCService tag.Key = tag.MustNewKey("grpc_service")
-	TagKeyGRPCMethod  tag.Key = tag.MustNewKey("grpc_method")
-	TagKeyHost        tag.Key = tag.MustNewKey("host")
-	TagKeyDestination tag.Key = tag.MustNewKey("destination")
+	TagKeyHTTPMethod  = tag.MustNewKey("http_method")
+	TagKeyService     = tag.MustNewKey("service")
+	TagKeyGRPCService = tag.MustNewKey("grpc_service")
+	TagKeyGRPCMethod  = tag.MustNewKey("grpc_method")
+	TagKeyHost        = tag.MustNewKey("host")
+	TagKeyDestination = tag.MustNewKey("destination")
 )
 
 // Default distributions used by views in this package.