Adds custom attribute store

[BernardGatt]

Adds a custom attribute store to forward custom attributes to the renderer

Part of: https://linear.app/customerio/issue/INAPP-13903/inject-anonymous-id-to-lead-form-submission

[jdcantrell]

Looks good, my only question is would we want to use sessionStorage over localStorage? I'm a little worried about us caching the wrong anonymous id, though maybe it's a non-issue if the gist SDK updates always override any existing value.

[BernardGatt]

Looks good, my only question is would we want to use sessionStorage over localStorage? I'm a little worried about us caching the wrong anonymous id, though maybe it's a non-issue if the gist SDK updates always override any existing value.

Its important we persist the id between tabs.