Fix bug with merging vault agent configs that set template_config

[blueberry-pi]

Description

This change fixes #29654, where giving the vault agent multiple config files can cause the resulting merged config to be incorrect when template_config is set in one of the config files. Specifically, when the first config file provided to the vault agent has a template_config stanza, and the second one doesn't, the merged config only has default template_config values. This is because we started creating a TemplateConfig struct even for configs that didn't specify it, to ensure that the MaxConnectionsPerHost default was being honored (#24989). But we were still doing a nil check for TemplateConfig during the merge process to determine if it needed to be updated, so we were overwriting it with the defaults if the config being processed didn't specify it.

I changed the nil check to checking against the default TemplateConfig value, and also moved setting that default from within parseTemplateConfig into NewConfig() to make sure the default continues to be set when no template_config is specified.

TODO only if you're a HashiCorp employee

• Backport Labels: If this fix needs to be backported, use the appropriate backport/ label that matches the desired release branch. Note that in the CE repo, the latest release branch will look like backport/x.x.x, but older release branches will be backport/ent/x.x.x+ent.
  • LTS: If this fixes a critical security vulnerability or severity 1 bug, it will also need to be backported to the current LTS versions of Vault. To ensure this, use all available enterprise labels.
• ENT Breakage: If this PR either 1) removes a public function OR 2) changes the signature
  of a public function, even if that change is in a CE file, double check that
  applying the patch for this PR to the ENT repo and running tests doesn't
  break any tests. Sometimes ENT only tests rely on public functions in CE
  files.
• Jira: If this change has an associated Jira, it's referenced either
  in the PR description, commit message, or branch name.
• RFC: If this change has an associated RFC, please link it in the description.
• ENT PR: If this change has an associated ENT PR, please link it in the
  description. Also, make sure the changelog is in this PR, not in your ENT PR.

[blueberry-pi]

Moving this here because now that we do check correctly below, the resulting TemplateConfig reverted back to being nil again when not set. So the default needs to be set in NewConfig() so it is in the base config we use when merging

[github-actions]

CI Results:
All Go tests succeeded! ✅

[github-actions]

Build Results:
All builds succeeded! ✅

[VioletHynes]

This looks great! Loving the new test, too. This should have a changlog describing the bug fix!