Backport of Fix bug with merging vault agent configs that set template_config into release/1.19.x

[hc-github-team-secure-vault-core]

Backport

This PR is auto-generated from #29680 to be assessed for backporting due to the inclusion of the label backport/1.19.x.

The below text is copied from the body of the original PR.

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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.
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Overview of commits

• b93e8e8

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