[API Review] Memory performance improvements

[RachitMalik12]

Currently we instantiate new objects every time for the following properties, this can have a large memory impact for large OpenAPI descriptions. For eg: For the Graph OpenAPI this will create 142k schemas, for a total of 1.1 million objects that might be unused. So we have an opportunity for some memory perf. improvements by lazily instantiating these.

 public virtual IDictionary<string, OpenApiSchema> PatternProperties { get; set; } = new Dictionary<string, OpenApiSchema>();
        public virtual IList<JsonNode> Enum { get; set; } = new List<JsonNode>();
        public virtual IList<OpenApiSchema> AllOf { get; set; } = new List<OpenApiSchema>();
        public virtual IList<OpenApiSchema> OneOf { get; set; } = new List<OpenApiSchema>();
        public virtual IList<OpenApiSchema> AnyOf { get; set; } = new List<OpenApiSchema>();
        public virtual ISet<string> Required { get; set; } = new HashSet<string>();
        public virtual IDictionary<string, OpenApiSchema> Properties { get; set; } = new Dictionary<string, OpenApiSchema>();
        public virtual IDictionary<string, IOpenApiExtension> Extensions { get; set; } = new Dictionary<string, IOpenApiExtension>();

A couple of different ideas were identified:

1. Lazy Instantiation: Explore using the init pattern and lazy get instantiation to reduce unnecessary object allocations in the OpenAPISchema
2. Structs vs. Classes: Evaluate the possibility of redefining OpenAPISchema and other types as structs instead of classes to improve performance.
3. IDictionary properties should have a default value and no setter.

[RachitMalik12]

Sample of documents needed:

1. Document that uses minimal features.
2. Document that is more complex. (eg: point in time snapshot (via submodule) graph beta openapi spec)
   Use these as tests to see if the changes actually improve performance.

[RachitMalik12]

We need to make a decision on whether we should use v1 as a baseline and then port the same testing to v2.

[baywet]

(assigned to myself to build the benchmarking and comparison infrastructure for this, Maggie should continue working on the performance improvements)

[RachitMalik12]

Something that was removed from the docs, putting it here for reference, related to your comment @baywet

Potential Performance improvements

        public virtual IDictionary<string, OpenApiSchema> PatternProperties { get; set; } = new Dictionary<string, OpenApiSchema>();
        public virtual IList<JsonNode> Enum { get; set; } = new List<JsonNode>();
        public virtual IList<OpenApiSchema> AllOf { get; set; } = new List<OpenApiSchema>();
        public virtual IList<OpenApiSchema> OneOf { get; set; } = new List<OpenApiSchema>();
        public virtual IList<OpenApiSchema> AnyOf { get; set; } = new List<OpenApiSchema>();
        public virtual ISet<string> Required { get; set; } = new HashSet<string>();
        public virtual IDictionary<string, OpenApiSchema> Properties { get; set; } = new Dictionary<string, OpenApiSchema>();
        public virtual IDictionary<string, IOpenApiExtension> Extensions { get; set; } = new Dictionary<string, IOpenApiExtension>();