[core] Add model cache to speed up code generation

Created by: sebastien-rosset

This is a performance enhancement. The code generation behavior does not change:

1. Create a cache for StringUtils.escape()
2. Create a cache from named model to CodegenModel. Used by fromModel()
3. Create a cache from named property to CodegenProperty. Used by fromProperty()
4. Create a cache from language-specific model name to Shema.
5. Pre-compile regex patterns instead of calling String.replaceAll().
6. Other minor optimizations

I've noticed for a large OpenAPI doc, the code generation was taking more than 3 minutes and 47% of all the CPU processing time was spent invoking the findFirst() method in PythonClientExperimentalCodegen.java (as shown below). This PR creates a map from toModelName() to schema such that toModelName() does not have to be called many time repeatedly and the Schema map does not have to be traversed repeatedly.

 Optional<Schema> referencedSchema = ModelUtils.getSchemas(openAPI).entrySet().stream()
                .filter(entry -> Objects.equals(varDataType, toModelName(entry.getKey())))
                .map(Map.Entry::getValue)
                .findFirst();

https://github.com/OpenAPITools/openapi-generator/blob/master/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/PythonClientExperimentalCodegen.java#L453

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