Allow casting between sync and async versions of TokenCredentials

**Is your feature request related to a problem? Please describe.**
A clear and concise description of what the problem is. Ex. I'm always frustrated when [...]
I have either a async or sync TokenCredential, for instance a AzureCliCredential, but in some cases I need the sync version, while in others I need the async.

For example:
Using the AIProjectClient from `azure.ai.projects.aio` (because using non-async with these kinds of services doesn't make sense) needs a async TokenCredential, but if I then want to configure azure monitor, I need a sync Credential to set that up, so I would need to either import and create credentials twice, or do some trickery to make a derived version myself.

**Describe the solution you'd like**
A clear and concise description of what you want to happen.
```python
from azure.identity.aio import AzureCliCredential

credential = AzureCliCredential()
sync_cred = credential.as_sync()
```
or the inverse:
```python
from azure.identity import AzureCliCredential

credential = AzureCliCredential()
async_cred = credential.as_async()
```

**Describe alternatives you've considered**
A clear and concise description of any alternative solutions or features you've considered.
```python
from azure.identity.aio import AsyncAzureCliCredential
from azure.identity import AzureCliCredential

credential = AzureCliCredential()
async_cred = AsyncAzureCliCredential.from_credential(credential)
```

**Additional context**
Add any other context or screenshots about the feature request here.