Support adding custom policies

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

I am trying to work around https://github.com/Azure/azure-sdk-for-python/issues/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:

https://github.com/Azure/azure-sdk-for-python/blob/ad5308d9befefdc0cdc61c4a47a3329fd874b8a0/sdk/network/azure-mgmt-network/azure/mgmt/network/_network_management_client.py#L72-L73

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

https://github.com/Azure/azure-sdk-for-python/blob/de7168a07ec7476f589f316d2cbff6f42f8fda91/sdk/core/azure-mgmt-core/azure/mgmt/core/_pipeline_client.py#L54-L69

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

https://github.com/Azure/azure-sdk-for-python/blob/3bf56880b6b8cc8ca7d783c96e4a5165eeb0212c/sdk/network/azure-mgmt-network/azure/mgmt/network/_configuration.py#L51-L66

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.

[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.

[xiangyan99]

Fixed.