add update_mask_fields to healthcare_hl7_v2_store parser_config

[modular-magician]

Fixes hashicorp/terraform-provider-google#9870

We were sending only top-level fields to updateMask for parser_config, but parser_config.version was immutable, so we needed to break it down to the nested fields.

TIL update_mask_fields

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Release Note Template for Downstream PRs (will be copied)

healthcare: fixed bug where changes to `google_healthcare_hl7_v2_store.parser_config` subfields would error with '...parser_config.version field is immutable...`

Derived from GoogleCloudPlatform/magic-modules#5122