Lookup schema with discriminator in OpenAPI snippets

[andrueastman]

This PR closes #629

It updates the OpenAPI snippet generator to receive the list of schemas in the reference document so as to allow for the lookup of schemas on snippet generation. This assists in the scenario where the node schema defines the base type (e.g. Attachment) but the payload defines a derived type(e.g. FileAttachement) with the properties from the derived type.
As the generator only has the types from the base type, it ends up placing the properties of the derived instance in the additionalDAta rather than initialize the derived type and use its propeties.

For example

var requestBody = new Attachment
{
	OdataType = "#microsoft.graph.fileAttachment",
	Name = "smile",
	AdditionalData = new Dictionary<string, object>
	{
		{
			"contentBytes" , "R0lGODdhEAYEAA7"
		},
	},
};

Should be

var requestBody = new FileAttachment
{
	OdataType = "#microsoft.graph.fileAttachment",
	Name = "smile",
	ContentBytes = Convert.FromBase64String("R0lGODdhEAYEAA7"),
};

As the changes affect the SnippetCodeGraph, the fix should apply for all languages using the SnippetCodeGraph and type information updated.

This PR also

• Refactors the test to use a common initializer to reduce code repetition in the test project.
• Fixes url template matching for paths that contain both indexers and functions and a function would be misinterpreted as an indexer due to order of processing. See docs at https://learn.microsoft.com/en-us/graph/api/callrecords-callrecord-getpstncalls?view=graph-rest-1.0&tabs=http

For easier review the PR is composed of 2 updates

• Update 1(Refactor tests to use common base)
• Update 2(Lookup Schema and other fixes)

Generation Diff

• https://github.com/microsoftgraph/microsoft-graph-docs/compare/preview-snippet-generation/117784...preview-snippet-generation/117818

Updates made

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

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88.3% Coverage
0.0% Duplication