Misc refactors involving MQTT5 needed for the MQTT3 adapter

[bretambrose]

• Removes const from client config storage so that the adapter's config APIs can mutate values inside
• Converts the connect packet storage in the MQTT5 client config from by-value to by-pointer. This allows us to more easily swap or mutate binary data within the connect storage.
• Removes a dangerous and unneeded ref-count acquire/release pair on the mqtt5 client during the websocket handshake transformation process. The mqtt5 client state machine does not allow interruption of the connecting state by the termination process, so there's no need to inc/dec and doing so exposes a race condition that could cause a second termination request.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.03 :warning:

Comparison is base (16b7c61) 81.20% compared to head (89b75c4) 81.17%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #284 +/- ## ========================================== - Coverage 81.20% 81.17% -0.03% ========================================== Files 18 18 Lines 7311 7311 ========================================== - Hits 5937 5935 -2 - Misses 1374 1376 +2 ``` | [Impacted Files](https://app.codecov.io/gh/awslabs/aws-c-mqtt/pull/284?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=awslabs) | Coverage Δ | | |---|---|---| | [source/v5/mqtt5\_client.c](https://app.codecov.io/gh/awslabs/aws-c-mqtt/pull/284?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=awslabs#diff-c291cmNlL3Y1L21xdHQ1X2NsaWVudC5j) | `88.34% <100.00%> (-0.02%)` | :arrow_down: | | [source/v5/mqtt5\_options\_storage.c](https://app.codecov.io/gh/awslabs/aws-c-mqtt/pull/284?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=awslabs#diff-c291cmNlL3Y1L21xdHQ1X29wdGlvbnNfc3RvcmFnZS5j) | `90.18% <100.00%> (+0.01%)` | :arrow_up: | ... and [1 file with indirect coverage changes](https://app.codecov.io/gh/awslabs/aws-c-mqtt/pull/284/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=awslabs)

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