Workflow: Cache ModelResolver and use cached FhirContext

[vitorpamplona]

ModelResolver is one of the slowest objects to load: 5.12 seconds in debug.

This PR

1. Makes R4FhirModelResolver a singleton.
2. Makes use of a cached context for increased performance.

The default constructor R4FhirModelResolver() constructor recreates the FhirContext.

IMPORTANT: All PRs must be linked to an issue (except for extremely trivial and straightforward changes).

Fixes #[issue number]

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• I have read and acknowledged the Code of conduct.
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• I have signed the Google Individual CLA, or I am covered by my company's Corporate CLA.
• I have discussed my proposed solution with code owners in the linked issue(s) and we have agreed upon the general approach.
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