OpenAPIService serverBaseUrl is not thread safe

[jmkelm08]

Describe the bug

I am trying to accomplish something similar to #2171. The recommended solution is to use a ServerBaseUrlCustomizer to customize the URL based on the request. In OpenAPIService.setServerBaseUrl, the customized base URL is stored in a member variable of the service with no locking or other protections which is not thread safe.

To Reproduce

1. Create a very basic Spring boot application
2. Run in debug
3. Set breakpoint 1 on the last line of OpenAPIService.setServerBaseUrl: this.serverBaseUrl = customServerBaseUrl
4. Set breakpoint 2 on the first line inside the if statement in OpenAPIService.updateServers
5. Open http://localhost:8080/swagger-ui/index.html
6. Advance to the 2nd breakpoint (in updateServers)
7. Open http://127.0.0.1:8080/swagger-ui/index.html
8. Step over the 1st breakpoin (last line in setServerBaseUrl)
9. Unpause the first request and see that the returned server list contains http://127.0.0.1:8080 instead of http://localhost:8080

Expected behavior

The server base URL is not modified by unrelated requests