Automate file headers

[niloc132]

Automates license headers and updates generated headers to more
clearly indicate what should be changed. Removes spotless exclusions
where possible, using file comments instead when necessary. Disables
spotless for files not under the dh license.

This will fail to build, the next commit, #5208 will apply these
changes across the codebase.

Fixes #1997

[devinrsmith]

Hard to do shallow review. Things look ok. Will try running commands. The // @formatter:off is much nicer than trying to finely specify the excludes.

[niloc132]

Tests are passing of the finished product: https://github.com/niloc132/deephaven-core/actions/runs/8162832698