Implement Synth as an exchange rate provider (#574)

* Implement Synth as an exchange rate provider

* Add assertions to provider interface test

* Assert the correct provider error is raised

* Remove unnecessary parens

Gemfile.lock

@@ -134,6 +134,9 @@ GEM
       xpath (~> 3.2)
     concurrent-ruby (1.2.3)
     connection_pool (2.4.1)
+    crack (1.0.0)
+      bigdecimal
+      rexml
     crass (1.0.6)
     date (3.3.4)
     debug (1.9.1)
@@ -164,6 +167,7 @@ GEM
       fugit (>= 1.1)
       railties (>= 6.0.0)
       thor (>= 0.14.1)
+    hashdiff (1.1.0)
     highline (3.0.1)
     hotwire-livereload (1.3.1)
       actioncable (>= 6.0.0)
@@ -214,6 +218,8 @@ GEM
     matrix (0.4.2)
     mini_mime (1.1.5)
     minitest (5.21.2)
+    mocha (2.1.0)
+      ruby2_keywords (>= 0.0.5)
     msgpack (1.7.2)
     net-http (0.4.1)
       uri
@@ -335,6 +341,7 @@ GEM
       ruby-lsp (>= 0.14.2, < 0.15.0)
       sorbet-runtime (>= 0.5.9897)
     ruby-progressbar (1.13.0)
+    ruby2_keywords (0.0.5)
     rubyzip (2.3.2)
     selenium-webdriver (4.18.1)
       base64 (~> 0.2)
@@ -371,11 +378,16 @@ GEM
     unicode-display_width (2.5.0)
     uri (0.13.0)
     useragent (0.16.10)
+    vcr (6.2.0)
     web-console (4.2.1)
       actionview (>= 6.0.0)
       activemodel (>= 6.0.0)
       bindex (>= 0.4.0)
       railties (>= 6.0.0)
+    webmock (3.23.0)
+      addressable (>= 2.8.0)
+      crack (>= 0.3.2)
+      hashdiff (>= 0.4.0, < 2.0.0)
     webrick (1.8.1)
     websocket (1.2.10)
     websocket-driver (0.7.6)
@@ -408,6 +420,7 @@ DEPENDENCIES
   inline_svg
   letter_opener
   lucide-rails!
+  mocha
   pagy
   pg (~> 1.5)
   propshaft
@@ -422,7 +435,9 @@ DEPENDENCIES
   tailwindcss-rails
   turbo-rails
   tzinfo-data
+  vcr
   web-console
+  webmock
 
 RUBY VERSION
    ruby 3.3.0p0