[Tables] Remove attribute support from TableEntity

[annatisch]

As discussed in design reviews, we've asserted that the TableEntity object will only be used as an output, and unlikely to be imported by customers to use as input. It should therefore be updated accordingly:

• Remove the _metadata key from inside the dictionary - this could conflict with a real entity property.
• Make sure metadata is read-only.
• Remove the custom attribute access from the object, so that properties within can only be accessed as dict key/values.
• Update input type hints to just Mapping[str, PrimitiveTypes]
• Look at whether we can insert TableEntity deserialization into the pipeline so as to avoid an extra dictionary construction (this is a perf consideration, and can come post-release).