Apply Flutter best practices [Non-Breaking] (#2)

* Add flutter_lints package

* Use key in widget constructors

lint: use_key_in_widget_constructors

* Unnecessary new keyword

lint: unnecessary_new

* Prefer const with constant constructors

* Exclude protobuf files from analyzer

* Private field could be final

* Annotate overridden members

* Use initializing formals when possible

* Don't access members with `this` unless avoiding shadowing

* Prefer is! operator

* Use isEmpty instead of length

* Use collection literals when possible

* Revert comment changes

* Cleaner null testing

* Misc fixes

* Avoid using `forEach` with a function literal

* Prefer `final` over `var` where applicable

* Enforce stricter type-checking

* Ignore VS Code files

* Flutter format

* Prefer using lowerCamelCase for constant names

* flutter format -l 100

* Disable `avoid_print` for examples

* Slight improvements to example

Should solve lint error: no_logic_in_create_state

lib/src/extensions.dart

@@ -4,17 +4,16 @@ class RTCConfiguration {
   String? iceTransportPolicy;
 
   Map<String, dynamic> toMap() {
-    var iceServersMap = [];
-    iceServers?.forEach((element) {
+    final iceServersMap = <Map<String, dynamic>>[];
+    for (final element in (iceServers ?? <RTCIceServer>[])) {
       iceServersMap.add(element.toMap());
-    });
-    return {
+    }
+    return <String, dynamic>{
       // only supports unified plan
       'sdpSemantics': 'unified-plan',
-      if (iceCandidatePoolSize != null)
-        "iceCandidatePoolSize": iceCandidatePoolSize,
-      "iceServers": iceServersMap,
-      if (iceTransportPolicy != null) "iceTransportPolicy": iceTransportPolicy,
+      if (iceCandidatePoolSize != null) 'iceCandidatePoolSize': iceCandidatePoolSize,
+      'iceServers': iceServersMap,
+      if (iceTransportPolicy != null) 'iceTransportPolicy': iceTransportPolicy,
     };
   }
 }
@@ -27,13 +26,22 @@ class RTCIceServer {
   RTCIceServer({required this.urls, this.username, this.credential});
 
   Map<String, dynamic> toMap() {
-    return {
-      "urls": urls,
-      if (username != null) "username": username,
-      if (credential != null) "credential": credential,
+    return <String, dynamic>{
+      'urls': urls,
+      if (username != null) 'username': username,
+      if (credential != null) 'credential': credential,
     };
   }
 }
 
-const RTCIceTransportPolicyAll = 'all';
-const RTCIceTransportPolicyRelay = 'relay';
+enum RTCIceTransportPolicy {
+  all,
+  relay,
+}
+
+extension RTCIceTransportPolicyExt on RTCIceTransportPolicy {
+  String toStringValue() => {
+        RTCIceTransportPolicy.all: 'all',
+        RTCIceTransportPolicy.relay: 'relay',
+      }[this]!;
+}