…returns cannot mutate underlying data

Reference: #556
Reference: #582

This will ensure that the `Attributes()`, `AttributeTypes()`, and `Elements()` methods return a copy of the underlying map or slice of data, rather than a direct reference to the map or slice. This also prevents `Object`-based plan modification from returning a panic since the updated `Object` `Attributes()` implementation will return an empty map instead of a `nil` map. Provider implementations should always rely on `IsNull()` and `IsUnknown()` for verifying whether types are known, rather than a nil comparison. This is considered a bug fix for the intended behavior of these type implementations rather than a breaking change as it standardizes type handling expectations.

New unit testing failures before code updates:

```
--- FAIL: TestMapValueElements_immutable (0.00s)
    /Users/bflad/src/github.com/hashicorp/terraform-plugin-framework/types/basetypes/map_test.go:604: unexpected Elements mutation

--- FAIL: TestNestedAttributeObjectPlanModify (0.00s)
    --- FAIL: TestNestedAttributeObjectPlanModify/response-planvalue-unknown-to-known-nested (0.00s)
panic: assignment to entry in nil map [recovered]
	panic: assignment to entry in nil map

goroutine 35 [running]:
testing.tRunner.func1.2({0x1010f0180, 0x10115b648})
	/opt/homebrew/Cellar/go/1.19.4/libexec/src/testing/testing.go:1396 +0x1c8
testing.tRunner.func1()
	/opt/homebrew/Cellar/go/1.19.4/libexec/src/testing/testing.go:1399 +0x378
panic({0x1010f0180, 0x10115b648})
	/opt/homebrew/Cellar/go/1.19.4/libexec/src/runtime/panic.go:884 +0x204
github.com/hashicorp/terraform-plugin-framework/internal/fwserver.NestedAttributeObjectPlanModify({_, _}, {_, _}, {{{0x1400010d190, 0x1, 0x1}}, {0x1, {0x1400010d1b0, 0x1, ...}}, ...}, ...)
	/Users/bflad/src/github.com/hashicorp/terraform-plugin-framework/internal/fwserver/attribute_plan_modification.go:1748 +0x510
```