chore(jmc-agent): clean up probe template utility

[tthvo]

Fixes #239

Changes include:

• addTemplate should check if template file exists before validating.
  • Add more contexts in case file-exist error occurs.
  • ProbeTemplate should correctly set its name.
• addTemplate should return the ProbeTemplate object so that the caller can directly call serialize on it.
• Input streams should properly close using the custom trulyClose.
• Rename methods to match returned values.

[tthvo]

I found that the template file is actually put into storage despite the API returning 400. This causes duplicate upload error when you try to upload the valid template with the same name.

After some digging, looks like validation (i.e. for the template specified in #238) is behaving differently between:

• Uploading templates (i.e. success)
  

  cryostat-core/src/main/java/io/cryostat/core/agent/LocalProbeTemplateService.java

  Line 90 in f386e39

  public void addTemplate(InputStream inputStream, String filename)

• Reading templates (i.e. failure in both methods below)
  

  cryostat-core/src/main/java/io/cryostat/core/agent/LocalProbeTemplateService.java

  Line 111 in f386e39

  public String getTemplate(String templateName) throws IOException, SAXException {

  
  

  cryostat-core/src/main/java/io/cryostat/core/agent/LocalProbeTemplateService.java

  Line 130 in f386e39

  public List<ProbeTemplate> getTemplates() throws FlightRecorderException {

With this PR changes, Cryostat will only call addTemplate when uploading. Hence, in the UI, upload will succeed so that attempts to send duplicate filename will fail with message about file already existing. This closes the issue in #239.

However, reloading the UI page will show the same issue in #238.

[maxcao13]

Testing with this and https://github.com/cryostatio/cryostat/pull/1563 seems to fix the issues!