chore: Extract `Event` into vector-core (#7240)

* chore: Extract `Event` into vector-core

This PR extracts `Event`, `metrics` and `mapping` module into vector-core. I had
hoped to do these separately but they proved to be tied together in a way I
couldn't separate. I've intentionally not made many structural changes to this
code. As much as possible it's a simple move operation.

Closes #7151 #7148

Signed-off-by: Brian L. Troutwine <brian@troutwine.us>

* address clippy warnings

Signed-off-by: Brian L. Troutwine <brian@troutwine.us>

* address failing vector-core tests

Signed-off-by: Brian L. Troutwine <brian@troutwine.us>

* clippy fixes

Signed-off-by: Brian L. Troutwine <brian@troutwine.us>

* correct log_schema

Signed-off-by: Brian L. Troutwine <brian@troutwine.us>

* correct panic! without formatting string

Signed-off-by: Brian L. Troutwine <brian@troutwine.us>

* address build issue after rebase

Signed-off-by: Brian L. Troutwine <brian@troutwine.us>