(GH-538) Define schema extension methods

[michaeltlombardi]

PR Summary

This change defines the SchemaUtilityExtensions trait in the dsc-lib-jsonschema crate to provide useful shorthand methods for the schemars::Schema type when munging and transforming schemas.

This change:

• Defines and implements the trait, providing documentation for every new method.
• Adds tests for the behaviors of every extension method.
• Depends on (GH-538) Add dsc-lib-jsonschema crate #1171

PR Context

While the trait includes some methods I expect to be useful, it doesn't try to fully map methods for retrieving every keyword. Instead, it primarily simplifies working with the following keywords:

• $id to retrieve and set the identifier for a schema.
• $defs to retrieve and munge referenced subschemas.
• properties to retrieve and munge property definitions.

The trait does define generic methods for retrieving keywords with an expected type, like get_keyword_as_object() and get_keyword_as_str. These are convenience methods to replace always calling code like the following when you want to retrieve a keyword with a known type:

let ref schema = json_schema!({
    "title": "Schema title"
});
if let Some(title) = schema.get("title").and_then(|v| v.as_str()) {
    println!("Schema title is {title}");
}

So you can instead do:

let ref schema = json_schema!({
    "title": "Schema title"
});
if let Some(title) = schema.get_keyword_as_str("title") {
    println!("Schema title is {title}");
}

[michaeltlombardi]

After #1171 is merged, I'll rebase this PR prior to final review and merge.

[SteveL-MSFT]

@michaeltlombardi can you rebase off main so I can see just the delta?

[michaeltlombardi]

@SteveL-MSFT - I don't think so, because these changes build on the dsc-lib-jsonschema crate.

But all the changes for this PR (on top of #1171) are in this commit:

970dd4e