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.

…ible with new implementation

…is is no longer required

…ntsinxmlconfig

The XML configuration provider now builds a simple in-memory model of the XML elements that it encounters
Each element keeps track of its attributes, children and content.
Furthermore, each element also has a reference to its siblings.
Each group of siblings all share the same list.

All of the above makes it possible to intelligently produce configuration keys and values, taking into account repeated XML elements.

… works as expected

Also don't explicitly initialize properties to null

…lement

…ntsinxmlconfig

Co-authored-by: Santiago Fernandez Madero <safern@microsoft.com>

Verify that a duplicate key exception is thrown when keys are duplicate with different casing
Verify that XML siblings are properly detected when the siblings have different casing
Verify that values with keys that were originally upper case can be retrieved with their lower case counterparts

This avoids confusion with the ProcessXYZ helper methods that run during a different stage in the algorithm

…sinxmlconfig