Backport changes in configuration.py, api_client.py, and rest.py to swagger-codegen

[mbohlool]

For velocity, we changed these files and disabled generation for them, but it is now time to backport changes to swagger-codegen so we can use the latest generator. These are items that need to be backported:

• config.http_proxy_url is now config.proxy (already exists in upstream). Release Note: config.http_proxy_url is deprecated. use configuration.proxy instead.
• (rest.py) port configuration.assert_hostname to upstream ([Python] Add configuration.{connection_pool_maxsize, assert_hostname} swagger-api/swagger-codegen#6508)
• (rest.py) port configuration.connection_pool_maxsize to upstream (with multiprocessing.cpu_count() * 5 default value) ([Python] Add configuration.{connection_pool_maxsize, assert_hostname} swagger-api/swagger-codegen#6508)
• (api_client.py) Upstream file does not have callback anymore, cleanup documentation in upstream about it ([Python] Add configuration.{connection_pool_maxsize, assert_hostname} swagger-api/swagger-codegen#6508)
• (api_client.py) Remove ws_client from this file. We probably need to make a separate utility like Watch for exec/attach calls (with its out ws_streaming_protocol config). Use stream module to call exec/attach calls #344
• (configuration.py) upstream configuration is not a singleton anymore, add a default configuration to upstream ([python] Add default Configuration swagger-api/swagger-codegen#6554).
• ws_client should not expect any customized config (such as ws_streaming_protocol) from configuration class.
• (requirements*.txt) let the generator generate these files and then add changes to main requirement files.
• (configuration.py) no upstream api_client config. validate the change for our client.
• consider enabling asyncio feature of python generator (support for asyncio #323, feat: add examples for asynchronous usage (callback, asycio) #324)
• update generator version to latest and validate generated client.

- config.http_proxy_url is deprecated. use configuration.proxy instead.
- Exec and attach calls does not return a streaming object anymore. Use kubernetes.stream instead
- Configuration is not a singleton object anymore. Please use Configuraion.set_default to change default configuration.
- Configuration class does not support `ws_streaming_protocol` anymore. Use ApiClient.set_default_header for `sec-websocket-protocol` to have the same effect.

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