Add support for a certificate file to be passed to `asyncssh.connect()`

[Andrew-S-Rosen]

This PR resolves #45 by adding a new cert_file kwarg and generalizing the client_keys kwarg passed to asyncssh.connect(). I have also cleaned up a few things related to the error-checking process when establishing the connection.

• [x] I have added the tests to cover my changes.
• [x] I have updated the documentation, VERSION, and CHANGELOG accordingly.
• [x] I have read the CONTRIBUTING document.

---

It looks like there are some issues with the unit tests getting started via the GitHub actions. I could use some assistance in addressing that.

[CLAassistant]

All committers have signed the CLA.

[Andrew-S-Rosen]

Thanks, @wjcunningham7! I'll address that shortly and let you know when I'm done.

If you need to merge #41 first, let me know and I'll address any merge conflicts here.

[Andrew-S-Rosen]

@wjcunningham7 --- Alright! I added the necessary tests and everything passes locally.

[codecov[bot]]

Codecov Report

Patch coverage: 43.75% and project coverage change: +2.87 :tada:

Comparison is base (f48c03d) 47.89% compared to head (177c4bd) 50.76%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #47 +/- ## =========================================== + Coverage 47.89% 50.76% +2.87% =========================================== Files 2 2 Lines 190 195 +5 =========================================== + Hits 91 99 +8 + Misses 99 96 -3 ``` | [Impacted Files](https://codecov.io/gh/AgnostiqHQ/covalent-slurm-plugin/pull/47?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=AgnostiqHQ) | Coverage Δ | | |---|---|---| | [covalent\_slurm\_plugin/slurm.py](https://codecov.io/gh/AgnostiqHQ/covalent-slurm-plugin/pull/47?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=AgnostiqHQ#diff-Y292YWxlbnRfc2x1cm1fcGx1Z2luL3NsdXJtLnB5) | `50.51% <43.75%> (+2.89%)` | :arrow_up: |

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[Andrew-S-Rosen]

@wjcunningham7 --- alright, should be ready to merge! Thanks for the help.