issue#2609 | Sasha | predictable axios requests (#2702)

* issue#2609 | Sasha | predictable axios requests

- axios requests are not delayed by pre-emptive promise creation by default
- add options to interceptors api ("synchronous" and "runWhen")
- add documentation and unit tests

* issue#2609 | Sasha | pull request feedback changes

* issue#2609 | Sasha | additional feedback changes

* issue#2609 | Sasha | put back try/catch

* issue#2609 | Sasha | add 2 adapter unit tests

- remove check for requestCancelled

Co-authored-by: ak71845 <alexandre.korotkov@kroger.com>
Co-authored-by: Xianming Zhong <chinesedfan@qq.com>
Co-authored-by: Jay <jasonsaayman@gmail.com>

README.md

@@ -590,6 +590,34 @@ const instance = axios.create();
 instance.interceptors.request.use(function () {/*...*/});
 ```
 
+When you add request interceptors, they are presumed to be asynchronous by default. This can cause a delay
+in the execution of your axios request when the main thread is blocked (a promise is created under the hood for 
+the interceptor and your request gets put on the bottom of the call stack). If your request interceptors are synchronous you can add a flag
+to the options object that will tell axios to run the code synchronously and avoid any delays in request execution.
+
+```js
+axios.interceptors.request.use(function (config) {
+  config.headers.test = 'I am only a header!';
+  return config;
+}, null, { synchronous: true });
+```
+
+If you want to execute a particular interceptor based on a runtime check, 
+you can add a `runWhen` function to the options object. The interceptor will not be executed **if and only if** the return
+of `runWhen` is `false`. The function will be called with the config
+object (don't forget that you can bind your own arguments to it as well.) This can be handy when you have an
+asynchronous request interceptor that only needs to run at certain times.
+
+```js
+function onGetCall(config) {
+  return config.method === 'get';
+}
+axios.interceptors.request.use(function (config) {
+  config.headers.test = 'special get headers';
+  return config;
+}, null, { runWhen: onGetCall });
+```
+
 ## Handling Errors
 
 ```js