[KYUUBI #6734] [DOC] add authentication example in REST API docs

# 🔍 Description
## Issue References 🔗

This pull request fixes #

## Describe Your Solution 🔧

-  add authentication example in REST API docs

## Types of changes 🔖

- [ ] Bugfix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing functionality to change)

## Test Plan 🧪

#### Behavior Without This Pull Request ⚰️

#### Behavior With This Pull Request 🎉

#### Related Unit Tests

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# Checklist 📝

- [ ] This patch was not authored or co-authored using [Generative Tooling](https://www.apache.org/legal/generative-tooling.html)

**Be nice. Be informative.**

Closes #6734 from bowenliang123/rest-doc-auth.

Closes #6734

f9ac944 [Cheng Pan] Update docs/client/rest/rest_api.md
528e55e [Bowen Liang] update doc
371af88 [Bowen Liang] update doc
e64a082 [Bowen Liang] update doc
341e7e0 [Bowen Liang] update doc

Lead-authored-by: Bowen Liang <[email protected]>
Co-authored-by: Cheng Pan <[email protected]>
Signed-off-by: Bowen Liang <[email protected]>

docs/client/rest/rest_api.md

@@ -19,6 +19,16 @@
 
 Note that: now the api version is v1 and the base uri is `/api/v1`.
 
+## Authentication
+
+REST API supports the [Basic Authentication](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Authorization#basic_authentication) which relies on `Authorization` header in HTTP request.
+
+```
+Authorization: Basic <credentials>
+```
+
+The `<credentials>` value is the Base64 encoded string of `username:password`. For example, in the case of a user `aladdin` with the password `opensesame`, set the `Authorization` header to `Basic YWxhZGRpbjpvcGVuc2VzYW1l` as Base64 encoded `aladdin:opensesame`.
+
 ## Session Resource
 
 ### GET /sessions