TAM: Introducing the level of Lead TAM

Why is this change being made?

We have more tenured TAMs on our team that delivering above and beyond the senior TAM role, to the level of what would be expected of a Lead TAM. We are looking to introduce this level to create clear guidelines and KPIs for future Lead TAMs

This MR replaces !69849 (closed) that has the discussion and feedback from the team.

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