Add a loader to enable jinja imports

[depoll]

Breaking change

Proposed change

Breaking out part of #88575

Enables importing/including shared jinja code/macros from a custom_jinja folder in your config folder.

All of this enables the following code:

In config/custom_jinja/shared.jinja:

{% macro some_macro() -%}
A macro
{%- endmacro %}
{%- set some_constant = "A constant" %}

Elsewhere:

{%- import "shared.jinja" as shared %}
{{ shared.some_constant }}
{{ shared.some_macro() }}

With the following output:

A constant
A macro

I'm happy to update the documentation as well but wanted to first clear that this would be a welcome change.

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New integration (thank you!)
• New feature (which adds functionality to an existing integration)
• Deprecation (breaking change to happen in the future)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

Additional information

• This PR fixes or closes issue: fixes #
• This PR is related to issue:
• Link to documentation pull request:

Checklist

• The code change is tested and works locally.
• Local tests pass. Your PR cannot be merged unless tests pass
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• Tests have been added to verify that the new code works.

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To help with the load of incoming pull requests:

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[balloob]

Duplicate of #85665

[depoll]

@balloob Thanks for the pointers. Quick question: if the issue is primarily I/O in the event loop, would it be ok to preload these files (in the given directory) into memory at startup or when a service is called to reload (like with automations) so that import never does I/O? Then we would use DictLoader in the template environment so that imports never go to disk.

[balloob]

Yeah, I think that could work. Let me give it some thoughts on how we would want this to be exposed to users.

[depoll]

Awesome. I have some prototype code (just wanted to see what it would take to load everything into memory -- very straightforward) updated in my branch that does this, though I haven't hooked it into any lifecycle yet (just happens once at startup at the moment). Happy to hack on it if you have suggestions for the shape it should take.