Fix invalid YAML Environment conversion when ?resolvePlaceholders=true

[zackman0010]

When converting an environment to YAML and resolving placeholders, the produced YAML will sometimes be invalid. Specifically, this happens when a placeholder is replaced with a string that starts with {, usually {cipher}. { is a reserved character in YAML and needs to be quoted.

When SnakeYAML sees a string that starts with {, it will print it as a single-quoted string with all other strings remaining unquoted:

parent:
  encrypted: '{cipher}1234567890abcdef'
  reference: ${parent.encrypted}

Spring will then take this produced String and resolve the properties in it, creating invalid YAML:

parent:
  encrypted: '{cipher}1234567890abcdef'
  reference: {cipher}1234567890abcdef # Invalid YAML

This PR fixes the issue by running a second post-process step to wrap any values that start with a { in single-quotes, matching the rest of the YAML structure. The below screenshot shows an example of the added line of code running.

[spencergibb]

Please add a test so we can guard against regressions

[zackman0010]

Please add a test so we can guard against regressions

I've added a regression test, but I wasn't able to run it. For some reason, org.springframework.boot.restclient is saying it doesn't exist even when using a fresh clone. I'm not sure if something is wrong on my system or if there's a dependency missing. Do you have any ideas?

[spencergibb]

Yes, it's a breaking change in boot, we will take care of it

[zackman0010]

After merging in the latest commit from main, I was able to run my test and confirm it was successful.