Spring Boot 2: StageName should be stripped from the path by default

*Serverless Java Container version*: 1.5.2

*Implementations:* Spring Boot 2

*Framework version:* spring-boot-starter-parent 2.3.8.RELEASE

*Frontend service:* AWS::Serverless::HttpApi (API Gateway HTTP)

*Deployment method:* SAM CLI

## Scenario
Configure a simple HTTP API proxy to Lambda using Spring Boot 2, and be able to route requests that ignore the stage-name.

## Expected behavior
Stage name (which is part of the path by default) is stripped before spring routes/maps to a controller.

## Actual behavior
Getting 404's b/c spring is mapping `/` vs ``; requires my code to be 'stage-name aware'.

## Steps to reproduce
event-source in sam template for AWS::Serverless::Function
```
Events:
BaseAdmin:
Type: HttpApi
Properties:
ApiId: !Ref Api
Method: any
Path: /{proxy+}
PayloadFormatVersion: "1.0"
RouteSettings:
ThrottlingBurstLimit: 100
ThrottlingRateLimit: 100
TimeoutInMillis: 29000
```
api def in sam template:
```
Api:
Type: AWS::Serverless::HttpApi
Properties:
FailOnWarnings: true
StageName: dev
```
lambda handler
```
public class LambdaHandler implements RequestStreamHandler {
private static final SpringBootLambdaContainerHandler HANDLER;

static {
Application.envSetup();
try {
HANDLER = new SpringBootProxyHandlerBuilder().defaultProxy()
.asyncInit()
.springBootApplication(Application.class)
.buildAndInitialize();
} catch (ContainerInitializationException e) {
e.printStackTrace();
throw new RuntimeException(e);
}
// Want to not need the line below uncommented to work!
// HANDLER.stripBasePath(System.getenv("STRIP_BASE_PATH"));
}

@Override
public void handleRequest(InputStream inputStream, OutputStream outputStream, Context context) throws IOException {
HANDLER.proxyStream(inputStream, outputStream, context);
}
}
```
## Full log output
*Paste the full log output from the Lambda function's CloudWatch logs*
`dev` is the stage name; the mapping is for /api/v1
```
DEBUG DispatcherServlet - GET "/dev/api/v1/some/resource", parameters={}
DEBUG DispatcherServlet - Exiting from "ERROR" dispatch, status 404
```

This isn't the worst thing ever...but it's definitely counter-intuitive to me. Would also appreciate any feedback on my solution; I did try some of the config options mentioned on the wiki page, but none of them seemed to do the trick until i found handler.stripBasePath. If there is a way to do this w/o passing in the StageName, that'd be better. Having it work by default would obviously be best.

從 LambdaHandler 及其 SpringBootProxyHandlerBuilder 設定開始，接著將 issue 中顯示的 SAM HttpApi StageName 和 proxy 路徑與 Spring Boot 收到的 request 路徑進行比較。檢查 handler.stripBasePath 的使用方式，並讓 stage 名稱預設不再需要；當像 /dev/api/v1/some/resource 這樣的 request 能以 /api/v1/some/resource 進行路由且不會回傳 404 時，即表示完成。

由索引模型根據 Issue 內容生成。