Cleaner client constructors

[melwil]

This PR aims to improve the constructor for new clients.

I've removed all code related to AsyncClient from client.py, and allowed you to directly make AsyncClient. This does mean it's a breaking change, but one that is easily caught by tests or type checkers.

There are a couple of reasons for this:

• When typing up the client (PR #68), the reasoning was that there should be less ambiguity about which types we are getting. Having new_client() making both types means that a typed project doesn't know if it has a Client or AsyncClient, and it has to be validated before use.
• Allowing developers to specifically choose sync or async clients is just better, especially if for some reason one would use both, so that type checkers can validate properly.
• Removed the environment variable support for making sync/async client, since it makes no sense to configure this on runtime. The clients are not interchangeable, you need to await async calls, and you either programmed for this or you didn't.

Finally, I will concede that it's not the greatest idea to repeat the code for getting the env vars and validating them in both constructors, it could be moved to some sort of utility function. I can do that if desireable, but in this case it's not a great chance for reusability.

[codecov-commenter]

Codecov Report

Attention: 9 lines in your changes are missing coverage. Please review.

Comparison is base (8b31f93) 77.08% compared to head (708a6cc) 76.83%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #92 +/- ## ========================================== - Coverage 77.08% 76.83% -0.25% ========================================== Files 27 27 Lines 1933 1943 +10 ========================================== + Hits 1490 1493 +3 - Misses 443 450 +7 ``` | [Files](https://app.codecov.io/gh/1Password/connect-sdk-python/pull/92?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None) | Coverage Δ | | |---|---|---| | [src/onepasswordconnectsdk/\_\_init\_\_.py](https://app.codecov.io/gh/1Password/connect-sdk-python/pull/92?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#diff-c3JjL29uZXBhc3N3b3JkY29ubmVjdHNkay9fX2luaXRfXy5weQ==) | `100.00% <100.00%> (ø)` | | | [src/tests/test\_client\_items.py](https://app.codecov.io/gh/1Password/connect-sdk-python/pull/92?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#diff-c3JjL3Rlc3RzL3Rlc3RfY2xpZW50X2l0ZW1zLnB5) | `100.00% <100.00%> (ø)` | | | [src/tests/test\_client\_vaults.py](https://app.codecov.io/gh/1Password/connect-sdk-python/pull/92?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#diff-c3JjL3Rlc3RzL3Rlc3RfY2xpZW50X3ZhdWx0cy5weQ==) | `100.00% <100.00%> (ø)` | | | [src/onepasswordconnectsdk/client.py](https://app.codecov.io/gh/1Password/connect-sdk-python/pull/92?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#diff-c3JjL29uZXBhc3N3b3JkY29ubmVjdHNkay9jbGllbnQucHk=) | `62.87% <66.66%> (-0.50%)` | :arrow_down: | | [src/onepasswordconnectsdk/async\_client.py](https://app.codecov.io/gh/1Password/connect-sdk-python/pull/92?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#diff-c3JjL29uZXBhc3N3b3JkY29ubmVjdHNkay9hc3luY19jbGllbnQucHk=) | `63.09% <38.46%> (-2.07%)` | :arrow_down: |

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[volodymyrZotov]

@melwil Agree on the changes you've introduced, that will make it work well for type checkers. However, it will be a breaching change, as in case somebody's already used the client with is_async parameter, this new version will break for them. But there is no problem with that, we can release v2.0.0 an mention this change in the release notes.

Could you resolve the conflict, please?