Add support for rust-analyzer

Fixes #2832

test/command_callback/test_rust_analyzer_callbacks.vader

@@ -0,0 +1,20 @@
+Before:
+  call ale#assert#SetUpLinterTest('rust', 'analyzer')
+
+After:
+  call ale#assert#TearDownLinterTest()
+
+Execute(The default executable path should be correct):
+  AssertLinter 'analyzer', ale#Escape('ra_lsp_server')
+
+Execute(The project root should be detected correctly):
+  AssertLSPProject ''
+
+  call ale#test#SetFilename('rust-rls-project/test.rs')
+
+  AssertLSPProject ale#path#Simplify(g:dir . '/rust-rls-project')
+
+Execute(Should accept configuration settings):
+  AssertLSPConfig {}
+  let b:ale_rust_analyzer_config = {'rust': {'clippy_preference': 'on'}}
+  AssertLSPConfig {'rust': {'clippy_preference': 'on'}}