Add support for provider-defined functions

[dbanck]

• Part of Provider-defined functions support vscode-terraform#1674

Background

We currently have a static list of all core functions in terraform-schema in the internal/funcs package. terraform-ls can consume this for a specific Terraform version via the FunctionsForVersion helper:

terraform-schema/schema/functions.go

Lines 20 to 24 in b462747

	func FunctionsForVersion(v *version.Version) (map[string]schema.FunctionSignature, error) {
	ver := v.Core()
	if ver.GreaterThanOrEqual(v1_4) {
	return funcs_generated.Functions(ver), nil
	}

To support provider-defined functions, we need to merge the built-in functions with the dynamic functions from the provider schema.

Proposal

• Add a Functions field to the provider schema to allow parsing from JSON
  

  terraform-schema/schema/provider_schema.go

  Lines 12 to 16 in b462747

  	type ProviderSchema struct {
  	Provider *schema.BodySchema
  	Resources map[string]*schema.BodySchema
  	DataSources map[string]*schema.BodySchema
  	}

• Add functions signatures to the new field when parsing provider JSON
• Introduce a new functions merger to combine the built-in functions with those defined by the provider. Similar to the existing SchemaMerger
  

  terraform-schema/schema/schema_merge.go

  Lines 37 to 41 in b462747

  	func NewSchemaMerger(coreSchema *schema.BodySchema) *SchemaMerger {
  	return &SchemaMerger{
  	coreSchema: coreSchema,
  	}
  	}