tfprovider.Provider adds a new method ImportManagedResourceState

[magodo]

Add a new method ImportManagedResourceState to tfprovider.Provider and implement it for both protocol5.Provider and protocol6.Provider.

Meanwhile, this commit fixes two bugs in existing code:

• protocol5.Provider and protocol6.Provider doesn't initialize the configureMu in their NewProvider method
• decodeDynamicValue in protocol5 and protocol6 packages incorrectly unmarshal the json payload when it actually should unmarshal msgpack

Test

I've locally used a demo code (just like the example code in this repo) to config the provider, import the resource and read it, it works at least for azurerm provider (plugin5):

💤  go run main.go ~/go/bin/terraform-provider-azurerm
cty.ObjectVal(map[string]cty.Value{"id":cty.StringVal("/subscriptions/xxxx/resourceGroups/zhwen-domain"), "location":cty.StringVal("westus"), "name":cty.StringVal("zhwen-domain"), "tags":cty.MapValEmpty(cty.String), "timeouts":cty.ObjectVal(map[string]cty.Value{"create":cty.NullVal(cty.String), "delete":cty.NullVal(cty.String), "read":cty.NullVal(cty.String), "update":cty.NullVal(cty.String)})})

[magodo]

@apparentlymart Would you mind take a look at this PR! Much appreaciated!