Add a property to support Brave's joint spans

[jonatan-ivanov]

Brave has a feature of joint spans that (supportsJoin(true)) was the default behavior with Spring Cloud Sleuth (and Boot 2.x). This has changed with Micrometer Tracing and Boot 3.x.
Overriding this in 3.x is not very simple since users need to create a Tracing bean to do this.

In order to make this easier and to support people who are migrating or just want this behavior, this flag could be set through a property.
(This is a Brave-only feature, OTel does not have this behavior.)

[jonatan-ivanov]

Closing in favor of #35165