Improve error message from SharedCredentialCacheCredential when mutltiple usernames are in the cache

[ellismg]

In #8095 we made some changes such that the SharedCredentialCacheCredential will raise an error when there are multiple identities in the cache and the user has not specified which to use. Today, this error lists all identities that we found, so the user can know what ones are available to select.

However - there a possible cases where we have multiple identities with the same username but multiple home_account_id's. The class does not presently support this case (since we don't have enough context to pick the right one) and if you try to use one of these identities, an error will be raised so we should consider listing only identities with a single home id to prevent users from setting the username to something that it turns out will not work.

ref:

azure-sdk-for-python/sdk/identity/azure-identity/azure/identity/_authn_client.py

Line 236 in de802a5

# TODO: we could identify usernames associated with exactly one home account id

azure-sdk-for-python/sdk/identity/azure-identity/azure/identity/aio/_authn_client.py

Line 90 in de802a5

# TODO: we could identify usernames associated with exactly one home account id

We expect that these cases are actually quite rare in practice.