[Cosmos DB] Add support for user identity to 'az cosmosdb identity assign/remove'

[zehende]

Description
This PR adds support for user identity in additional to system manages service identity, along with the relevant tests.

The parameter '--identities' has been added to the commands 'az cosmosdb identity assign' and 'remove' to accept a space-separated list of identities, which, in addition to the existing '[system]' for system-managed identity, supports user-managed identities.

Examples:

• Assign an existing user identity to a cosmos db account
  az cosmosdb identity assign -g myrg -n mycdbact --identities /subscriptions/00000000-0000-0000-00000000/resourcegroups/MyRG/providers/Microsoft.ManagedIdentity/userAssignedIdentities/MyID

• Remove a user identity from a cosmos db account
  az cosmosdb identity remove -g myrg -n mycdbact --identities /subscriptions/00000000-0000-0000-00000000/resourcegroups/MyRG/providers/Microsoft.ManagedIdentity/userAssignedIdentities/MyID

• Add system managed identity and user identity to a cosmos db account
  az cosmosdb identity assign -g myrg -n mycdbact --identities [system] /subscriptions/00000000-0000-0000-00000000/resourcegroups/MyRG/providers/Microsoft.ManagedIdentity/userAssignedIdentities/MyID

• Update the default identity to user identity
  az cosmosdb update -g myrg -n mycdbact ... --default-identity UserAssignedIdentity=/subscriptions/00000000-0000-0000-00000000/resourcegroups/MyRG/providers/Microsoft.ManagedIdentity/userAssignedIdentities/MyID

Testing Guide
azdev test cosmosdb

[yonzhan]

Cosmos DB

[xujin-zhang]

Hi, can you extend the description like this:
#17583

So it makes the commands sets more clear. Thanks a lot!

[j-patrick]

I added one comment, would like to see the test I can't find, then I will sign off. :)