Support adding custom policies

[jiasli]

Is your feature request related to a problem? Please describe.

I am trying to work around #8313 by adding a new custom policy called ExternalBearerTokenCredentialPolicy, but I find it difficult.

The only way to change the default policy list is to override the whole list by passing policies in ARMPipelineClient's kwargs:

azure-sdk-for-python/sdk/network/azure-mgmt-network/azure/mgmt/network/_network_management_client.py

Lines 72 to 73 in ad5308d

	self._config = NetworkManagementClientConfiguration(credential, subscription_id, **kwargs)
	self._client = ARMPipelineClient(base_url=base_url, config=self._config, **kwargs)

When ARMPipelineClient is created without policies argument, it builds the default policy list from config=self._config:

azure-sdk-for-python/sdk/core/azure-mgmt-core/azure/mgmt/core/_pipeline_client.py

Lines 54 to 69 in de7168a

	def _default_policies(config, **kwargs):
	return [
	RequestIdPolicy(**kwargs),
	ARMAutoResourceProviderRegistrationPolicy(),
	config.headers_policy,
	config.user_agent_policy,
	config.proxy_policy,
	ContentDecodePolicy(**kwargs),
	config.redirect_policy,
	config.retry_policy,
	config.authentication_policy,
	config.custom_hook_policy,
	config.logging_policy,
	DistributedTracingPolicy(**kwargs),
	config.http_logging_policy or ARMHttpLoggingPolicy(**kwargs),
	]

In order to add a custom policy to the default policy list, I must also manually create all policies created by NetworkManagementClientConfiguration:

azure-sdk-for-python/sdk/network/azure-mgmt-network/azure/mgmt/network/_configuration.py

Lines 51 to 66 in 3bf5688

	def _configure(
	self,
	**kwargs # type: Any
	):
	# type: (...) -> None
	self.user_agent_policy = kwargs.get('user_agent_policy') or policies.UserAgentPolicy(**kwargs)
	self.headers_policy = kwargs.get('headers_policy') or policies.HeadersPolicy(**kwargs)
	self.proxy_policy = kwargs.get('proxy_policy') or policies.ProxyPolicy(**kwargs)
	self.logging_policy = kwargs.get('logging_policy') or policies.NetworkTraceLoggingPolicy(**kwargs)
	self.http_logging_policy = kwargs.get('http_logging_policy') or ARMHttpLoggingPolicy(**kwargs)
	self.retry_policy = kwargs.get('retry_policy') or policies.RetryPolicy(**kwargs)
	self.custom_hook_policy = kwargs.get('custom_hook_policy') or policies.CustomHookPolicy(**kwargs)
	self.redirect_policy = kwargs.get('redirect_policy') or policies.RedirectPolicy(**kwargs)
	self.authentication_policy = kwargs.get('authentication_policy')
	if self.credential and not self.authentication_policy:
	self.authentication_policy = policies.BearerTokenCredentialPolicy(self.credential, *self.credential_scopes, **kwargs)

This apparently seems too complicated.

Describe the solution you'd like

SDKs should expose the ability to add custom policies to the default policy list.

[xiangyan99]

The problem here is the policy order does matter.

When users want to add a policy into the list, they have to tell the pipeline where they want the policy to be into.

That's the reason we ask users to give the entire list of policies. Any ideas?

[lmazuel]

As some point @johanste was considering a simple "first" "last" insert point, since this would solve most of the scenario.s

[xiangyan99]

As some point @johanste was considering a simple "first" "last" insert point, since this would solve most of the scenario.s

Then users need to decide they want to lose retry or (partial)logging. :(

[johanste]

"first" and "last" does not have to mean "literally first or last in the pipeline". "Last" can be synonymous with "do-this-for-each-try" and the first can be "do-this-once". The names can be improved.

One (more complicated) alternative is to give policies "well known names" and have an "insert before/after" or "replace" method for it. But that is probably more power than most people would need.