[Lora][Frontend]Add default local directory LoRA resolver plugin.

[jberkhahn]

This is my conversion of the implementation of #14634 into a plugin for the framework added in #10546.

This plugin with be "installed" by default, but LoRA resolvers will only run if VLLM_ALLOW_RUNTIME_LORA_UPDATING is set to True, and then on top of that this resolver with only register itself if VLLM_PLUGIN_LORA_CACHE_DIR is set to a local directory that exists.

FIX #12174

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[jberkhahn]

hmm, looks like the test_completion unit test isn't working because the plugin isn't getting picked up :/

[jeejeelee]

You can try sync the main branch to avoid the CI failure

[jberkhahn]

@jeejeelee rebased it again, that doesn't seem to have fixed it?

[jeejeelee]

It doesn't matter, if these failures are not related to this PR, we can consider force merging it.