AWS EC2 - Automatically chosen subnet does not match parametrized AZ

**Describe the issue**:

`ec2.py` [chooses the first subnet](https://github.com/dask/dask-cloudprovider/blob/5babf642835603fa354054176e096ea9eb68e971/dask_cloudprovider/aws/ec2.py#L98C1-L99C1) from the specified (or default, if none specified) VPC, ignoring the `availability_zone`(AZ) parameter.

Some VMs [are not supported on all AZs](https://repost.aws/knowledge-center/ec2-instance-type-not-supported-az-error), so it is necessary to provide an AZ, but doing so may conflict with the subnet selected on the step described above.

**Minimal Complete Verifiable Example**:

At the time of writing `m5.large` instance is not supported on `us-east-1e`, and that is the subnet returned at index 0 when listing subnets from default VPC in my case.

To better reproduce we can force use of the AZ of second subnet, creating this code:

```python
from dask_cloudprovider.aws.ec2 import EC2Cluster
from dask_cloudprovider.aws.helper import (
get_default_vpc,
)
from aiobotocore.session import get_session
import botocore

boto_config = botocore.config.Config(retries=dict(max_attempts=10))
region = "us-east-1"
async with get_session().create_client("ec2", region_name=region, config=boto_config) as client:
vpc = await get_default_vpc(client)
subnets = (await client.describe_subnets())["Subnets"]
az = subnets[1]['AvailabilityZone'] # Code at `dask_cloudprovider.aws.ec2` gets subnet at [0], so this will force the issue
EC2Cluster(
region="us-east-1",
availability_zone=az,
security=False, # Simply to avoid requiring criptography package
scheduler_instance_type="m5.large",
worker_instance_type="m5.large",
)
```

This outputs:

```log
2024-06-09 14:46:30,639 - distributed.deploy.spec - WARNING - Cluster closed without starting up
```

And inspecting the stack we can see the following error:
```log
[...]
ClientError: An error occurred (InvalidParameterValue) when calling the RunInstances operation: Value (us-east-1a) for parameter availabilityZone is invalid. Subnet '' is in the availability zone us-east-1e

During handling of the above exception, another exception occurred:
[...]
```

**Anything else we need to know?**:

Changing `dask_cloudprovider.aws.helper.get_vpc_subnets` to receive and consider the AvailabilityZone should fix the issue. If this makes sense I could open a PR! :)

```py
async def get_vpc_subnets(client, vpc, availability_zone):
vpcs = (await client.describe_vpcs())["Vpcs"]
[vpc] = [x for x in vpcs if x["VpcId"] == vpc]
subnets = (await client.describe_subnets())["Subnets"]
return [subnet["SubnetId"] for subnet in subnets if subnet["VpcId"] == vpc["VpcId"] and subnet["AvailabilityZone"] == availability_zone]
```

**Environment**:

- Dask version:
- dask==2024.4.1
- dask-cloudprovider==2022.10.0
- distributed==2024.4.1
- Python version: 3.10.12
- Operating System: Ubuntu 20.04 LTS (Linux 5.15.146.1-microsoft-standard-WSL2)
- Install method (conda, pip, source): pip