Grouping of BYOK custom models in model picker (#1111)

* used https://github.com/microsoft/vscode-copilot-chat/commit/87451aabfc2839ac48bc111094bcae6f46288632#diff-bcf7cff25eec039e0563b151a1299f0ad1f483594076c6bb649e2132e57d07e8 as starting point

* Update orgName to ownerName

* lets remove the ownerName if its there

* clean touched lines

* Make Copilot agree that cusomModel is not null inside the block. And easier to read

* Use same type based on review feedback

* 1 type now, so simpler:

* Update tooltip with better descriptions