fetcher: Rework API to use strings for tls keys/db

This is prep for the libcurl porting. `GTlsCertificate/GTlsDatabase` are
abstract classes implemented in glib-networking for gnutls. curl's APIs take
file paths as strings, so it's easier to work on both if we move the GLib TLS
bits into the libsoup code.

Closes: #651
Approved by: giuseppe

src/libostree/ostree-tls-cert-interaction.h

@@ -34,6 +34,7 @@ typedef struct _OstreeTlsCertInteractionClass   OstreeTlsCertInteractionClass;
 
 GType                       _ostree_tls_cert_interaction_get_type    (void) G_GNUC_CONST;
 
-OstreeTlsCertInteraction *  _ostree_tls_cert_interaction_new         (GTlsCertificate *cert);
+OstreeTlsCertInteraction *  _ostree_tls_cert_interaction_new         (const char *cert_path,
+                                                                      const char *key_path);
 
 G_END_DECLS