File store support of symmetric keys

The pull request https://github.com/openssl/openssl/pull/28278 as a part of series of changes to introduce a full support of symmetric keys implements the support of reading symmetric keys from raw files. The maximum size of the key is arbitrary limited to 2048 bytes, and to prevent automatic interpretation of any file to be a symmetric key instead of some structured file we need to explicitly say that we are reading it as raw bytes.

I would like to see committers' opinion on this. The PR is labeled by commiters' hold as of now.