[PJRT:C] Rename `PJRT_Structure_*` to `PJRT_Extension_*`
This is to make it more clear what the APIs pertain to (extensions to
the PJRT C API). This is a source-incompatible but ABI compatible
change. It should only affect plugins that use extensions.
This is part of the PJRT C API extension improvements described in
https://docs.google.com/document/d/1PsSIkCbHujPfiZphyQ3U5wB9VGBLKBjxGrTtmziJwEw/edit?usp=sharing
PiperOrigin-RevId: 606802550
