[Storage] added SessionProvider to create session

[anjaliratnam-msft]

Summary
This PR introduces SessionProvider building off the previous session PR. Customers can optionally supply one via session_options.session_provider and reuse it across clients to share the cache. If it's not supplied, one is created and scoped to the client, so existing behavior is unchanged.

Not in this PR

• Did not implement the async changes
• SessionOptions.account_name. The optional signing override, with URL extraction as fallback and the custom-endpoint caveat documented.
• Test updates. The existing session test reaches into internals that moved.

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