Document HttpClient usage

[thibauult]

The BDK proposes a generic HttpClient that allows developers to perform calls to external systems.

The advantage of this HttpClient is it relies on the same implementations that are used to perform calls to the Symphony REST APIs.

Also, developers are free to choose between different implementations by simply selecting the right dependency:

<dependency>
   <groupId>org.finos.symphony.bdk</groupId>
   <artifactId>symphony-bdk-http-jersey2</artifactId>
   <scope>runtime</scope>
</dependency>
<!-- OR -->
<dependency>
   <groupId>org.finos.symphony.bdk</groupId>
   <artifactId>symphony-bdk-http-webclient</artifactId>
   <scope>runtime</scope>
</dependency>

What do we want?

• create new http-client.md in ./docs/ folder
• reference this file in index.md
• document configuration and usage of the HttpClient interface and its different implementations

[mcleo-d]

Thank you @symphony-thibault for your very well articulated good first issue. I have added to This Week at FINOS to help drive engagement from the FINOS Community 🚀