(GH-538) Define newtypes for exit_codes fields

[michaeltlombardi]

PR Summary

This change defines two new types:

• ExitCode as a lightweight wrapper around i32 where we can control serialization and deserialization with strings as the serialized format instead of an integer.

• ExitCodesMap as a lightweight wrapper for HashMap<ExitCode, String>. It implements a helper function for determining whether the map is empty or default and defines the default map for exit 0 (success) and 1 (error).

  The ExitCodesMap includes a hand-implementation of JsonSchema to match the canonical JSON Schema, which schemars won't generate from the struct definition.

This change not only enables us to provide the canonical JSON Schema for exit code fields in a reusable way, it also removes the need to maintain a converter function for transforming the exit codes map from HashMap<String, String> to HashMap<i32, String>.

This change also:

• Updates the ResourceManifest and ExtensionManifest structs to use the ExitCodesMap for the exit_codes field.
• Updates the implementations of run_process_async() and invoke_command() to use the new type.
• Removes the now-erroneous convert_hashmap_string_keys_to_i32() function, since the exit codes are correctly typed from deserialization/instance creation.

Note

Leaving this in draft until #1350 is merged, as it includes new development dependencies useful for quickly defining test cases for the newtypes.

PR Context

Prior to this change, the exit_codes fields for the resource and extension manifests defined the map of exit codes to their semantic meanings as Option<HashMap<String, String>>.

As part of schema canonicalization, we need to provide a canonical JSON Schema for that object. Unfortunately, because we don't own either the JsonSchema trait or HashMap<T> type, we need to use a newtype wrapper.