Add support for setting the value in "using"

[rsabirov]

Some sink implementations using the name of sink name which is different from namespace. To support this we need to introduce value in using part of the config.

Here is example for https://github.com/saleem-mirza/serilog-sinks-azure-analytics.

app.config way of configuring:

<appSettings>
  <add key="serilog:using:AzureLogAnalytics" value="Serilog.Sinks.AzureAnalytics" />
 </appSettings>

Expected xml way of configuring:

<serilog>
  <using>
    <add key="AzureLogAnalytics" value="Serilog.Sinks.AzureAnalytics"/>
  </using>
</serilog>

Having only value should be still supported:

<serilog>
  <using>
    <add value="Serilog.Enrichers.Thread" />
    <add value="Serilog.Enrichers.Process" />
    <add value="Serilog.Sinks.Seq" />
  </using>
</serilog>