Reorganize the plugins directory to group by plugin name rather than extension

[ahg-g]

What would you like to be added:

Reorganize the plugins directory to group by plugin name rather than extension

Why is this needed:
It makes it easier to navigate the code to look for a specific feature. I suggest the following structure:

plugins/pd
plugins/activerequestscorer
plugins/preciseprefixcachescorer
...

[elevran]

kubernetes-sigs/gateway-api-inference-extension#1889 for additional context on upstream