Support non-string computed keys

[dlecocq]

Dealing with an API with return values that are non-string scalars, and I need to be able to consume them as part of this provider. Specifically, an API with an id that is getting returned as a number, which is causing this trace:

goroutine 14 [running]:
github.com/sullivtr/terraform-provider-graphql/graphql.computeMutationVariableKeys(0xc0001c4200, 0x1762dcf)
	github.com/sullivtr/terraform-provider-graphql/graphql/keys.go:36 +0x18a
github.com/sullivtr/terraform-provider-graphql/graphql.computeMutationVariables({0xc0001c4200, 0x31, 0x200}, 0x1663040)
	github.com/sullivtr/terraform-provider-graphql/graphql/resource_graphql_mutation.go:327 +0x186
github.com/sullivtr/terraform-provider-graphql/graphql.executeCreateHook({0x184c568, 0xc000054ea0}, 0x0, {0x16473e0, 0xc00009ff60})
	github.com/sullivtr/terraform-provider-graphql/graphql/resource_graphql_mutation.go:273 +0x29f
github.com/sullivtr/terraform-provider-graphql/graphql.resourceGraphqlMutationCreate({0x184c568, 0xc000054ea0}, 0xc0005597b8, {0x16473e0, 0xc00009ff60})
	github.com/sullivtr/terraform-provider-graphql/graphql/resource_graphql_mutation.go:128 +0x45
github.com/hashicorp/terraform-plugin-sdk/v2/helper/schema.(*Resource).create(0xc0000b8380, {0x184c4f8, 0xc0002842c0}, 0x2, {0x16473e0, 0xc00009ff60})
	github.com/hashicorp/terraform-plugin-sdk/v2@v2.7.0/helper/schema/resource.go:330 +0x12e
github.com/hashicorp/terraform-plugin-sdk/v2/helper/schema.(*Resource).Apply(0xc0000b8380, {0x184c4f8, 0xc0002842c0}, 0xc0003362a0, 0xc00009e700, {0x16473e0, 0xc00009ff60})
	github.com/hashicorp/terraform-plugin-sdk/v2@v2.7.0/helper/schema/resource.go:456 +0x871
github.com/hashicorp/terraform-plugin-sdk/v2/helper/schema.(*GRPCProviderServer).ApplyResourceChange(0xc0000a6780, {0x184c4f8, 0xc0002842c0}, 0xc000312500)
	github.com/hashicorp/terraform-plugin-sdk/v2@v2.7.0/helper/schema/grpc_provider.go:955 +0x9aa
github.com/hashicorp/terraform-plugin-go/tfprotov5/server.(*server).ApplyResourceChange(0xc00009efc0, {0x184c5a0, 0xc0004a6270}, 0x101732b)
	github.com/hashicorp/terraform-plugin-go@v0.3.0/tfprotov5/server/server.go:332 +0x6c
github.com/hashicorp/terraform-plugin-go/tfprotov5/internal/tfplugin5._Provider_ApplyResourceChange_Handler({0x1729e00, 0xc00009efc0}, {0x184c5a0, 0xc0004a6270}, 0xc000054660, 0x0)
	github.com/hashicorp/terraform-plugin-go@v0.3.0/tfprotov5/internal/tfplugin5/tfplugin5_grpc.pb.go:380 +0x170
google.golang.org/grpc.(*Server).processUnaryRPC(0xc0000ce700, {0x1858df0, 0xc00019c480}, 0xc0000b2300, 0xc0004226c0, 0x1ce1c60, 0x0)
	google.golang.org/grpc@v1.32.0/server.go:1194 +0xc8f
google.golang.org/grpc.(*Server).handleStream(0xc0000ce700, {0x1858df0, 0xc00019c480}, 0xc0000b2300, 0x0)
	google.golang.org/grpc@v1.32.0/server.go:1517 +0xa2a
google.golang.org/grpc.(*Server).serveStreams.func1.2()
	google.golang.org/grpc@v1.32.0/server.go:859 +0x98
created by google.golang.org/grpc.(*Server).serveStreams.func1
	google.golang.org/grpc@v1.32.0/server.go:857 +0x294

Error: The terraform-provider-graphql_2.5.4 plugin crashed!

[dlecocq]

I'm trying to run the tests locally, but the makefile isn't working for me (yet). In the meantime, I'm pushing this MR hoping that either 1) automated tests kick off or 2) it's quick enough for someone with a working environment to run the tests to verify.

[dlecocq]

Ah, was able to get the tests working well enough locally to demonstrate that the tests fail without the fix commit, and pass with it.