fix(custom-targets): lock form Create button until successful test

[andrewazores]

• fix(custom-targets): handle case where response does not include data or reason fields
• fix(custom-targets): lock form Create button until successful test

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Fixes: #1261

Description of the change:

• handles the case where the response body does not have .data.reason fields. This can happen if the server encounters an unexpected error and responds with an HTTP 500, in which case the response format is not a Cryostat API V2 format but a standard Quarkus 500 format.
• locks the Custom Target creation form Create button until a successful connection test is performed. If the form is updated (URL or credentials) the button is re-locked.

Motivation for the change:

When creating a custom target the server will already perform a connection test anyway. However, the UI is more informative and makes more sense if the user is first required to perform a successful connection test before they can create the target definition.

How to manually test:

1. Check out and build cryostat3 with this PR
2. ./smoketest.bash -Otr
3. Try to create some custom target definitions, like cryostat3:9091, localhost:0, sample-app-1:9093, reports:11124, jfr-datasource:11223, cryostat:9091

[github-actions[bot]]

Test image available:

CRYOSTAT_IMAGE=ghcr.io/cryostatio/cryostat-web:pr-1262-0ed25c938de2425a66ea3bcecc523f9bb77a087a bash smoketest.bash # then open http://localhost:8080