LocalFileStore should support a shared CacheBackedEmbeddings for multiple users

[chrispy-snps]

Checked

• I searched existing ideas and did not find a similar one
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• I've clearly described the feature request and motivation for it

Feature request

The LocalFileStore class can be used to create an on-disk embedding cache. However, the default umask settings gives file/directory write perrmissions only to the original user. Once the cache directory is created, other users cannot write their own cache entries into the directory.

To make the cache usable by multiple users, the LocalFileStore constructor should accept a umask parameter that is used for file/directory creation.

Motivation

We would like to share a CacheBackedEmeddings store between a small number of users. This enhancement would allow the following:

• When configured with a umask of 0o007 (user and group permitted, others masked), anyone in our group can contribute embeddings to the cache.
• If we implement cache cleanup/eviction in the future, anyone in our group can perform the cleanup.

Proposal (If applicable)

Update the LocalFileStore class to accept a umask parameter that is used for file/directory creation. If the umask parameter is not specified, the default behavior should be unchanged from today's behavior.

[chrispy-snps]

I filed #18077 to implement this.

[eyurtsev]

I don't think we want to extend functionality for the local file store. it's mostly there as a reference implementation / for easy prototyping locally.