For Azure/autorest.python#2795.

Compared to the previous logic that only supported a single namespace, this PR introduces support for multiple namespaces. This primarily requires updates in two key areas:

1. Serialization approach: We need to account for all namespaces and serialize them individually.
2. Relative imports: The Python SDK utilizes relative imports internally, and we can no longer make assumptions about the location of models/operations. To determine the correct relative import, we need to know the serialized namespace within the imports function.

Considering these two main changes, here's a detailed summary of the updates in this PR:

1. Added the emitter flag enable-typespec-namespace, which lets users decide if they want to support native typespec namespace.
2. Introduced clientNamespace for models/operations/enums/namedUnions/clients: the emitter uses clientNamespace to determine the exact location where the target type should be.
3. Created get_relative_import_path for CodeModel: all relative import paths should be calculated using this function.
4. Added a new property client_namespace_types to CodeModel: it counts and stores all namespace types and information about clients/operation_groups/enums/models in each namespace.
5. Serialization: With the new client_namespace_types property in CodeModel, we can cycle through each namespace and serialize its clients/models/enums/operations using the same logic.
6. Imports: The old import logic used the relative_path and operation kwargs, which were not set from the top caller but from various functions during the calling process. The signatures were difficult to understand and maintain. I replaces them with the new serialize_namespace and serialize_namespace_type kwargs. Now, any function that needs to calculate relative imports can use these two parameters, which are set from the top caller, making them easier to understand and maintain.