Timezone aware datetimes break queries

**Describe the bug**
cornflow client api_for_id method doesn't work with a datetime argument with a timezone.

The problem is that when transformed to string, the timezone is shown a "+xx:00"

In the url, the "+" is interpreted as a space.
The "+" in the url should be transformed into "%2B"

Currently, api_for_id is processing the arguments that way:
```
if query_args is not None:
url = f"{url}?"
for key, value in query_args.items():
url = f"{url}{key}={value}&"
url = url[:-1]
```

**To Reproduce**
Steps to reproduce the behavior:
1. Choose any endpoint which use a datetime argument
2. create a timezone aware date with my_date = datetime.now(timezone.utc)
3. use cornflow client api_for_id with query_args = {"date": my_date}
4. The schema check for the date argument will fail.

**Expected behavior**
api_for_id and other api functions should checks if the arguments are dates and in that case replace the +.

Otherwise, the + should not be replaced because it might be intentional.

**Metadata (please complete the following information):**
- cornflow version 1.2.1

Locate the client api_for_id method and the other API functions that build query strings from query_args. Reproduce the issue with datetime.now(timezone.utc) and check the schema validation result; done means timezone-aware datetime arguments preserve the encoded plus sign without altering intentional plus signs in other values.

Written by the indexing model from the issue text.