This commit adds support in Microsoft.Extensions.Configuration.Xml for repeated XML elements without requiring a Name attribute.
This solves a particularly subtle bug when configuring Serilog from an XML configuration source. For a full description, see dotnet#36541

The original implementation of the XmlStreamConfigurationProvider has been modified to do the following:

- Maintain a stack of encountered XML elements while traversing the XML source. This is needed to detect siblings.
- When siblings are detected, automatically append an index to the generated configuration keys. This makes it work exactly the same as the JSON configuration provider with JSON arrays.

Tests are updated to reflect the new behavior:
- Tests that verified an exception occurs when entering duplicate keys have been removed. Duplicate keys are supported now.
- Add tests that verify duplicate keys result in the correct configuration keys, with all the lower/upper case variants and with support for the special "Name" attribute handling.

This ensures that, at the very least, it doesn't crash
Correct behavior is covered by other tests