Add physics overrides for walk speed and Fast Mode (adoption)

[grorp]

This PR is an adoption of #13815 by Wuzzy2. I rebased and fixed a bug in the backwards compatibility code.

This PR adds the following new fields to set_physics_override:

• speed_walk: Normal walking speed
• speed_fast: Speed in Fast Mode
• acceleration_fast: Acceleration in Fast Mode

With this PR, every player physics value (movement_ settings) will be overridable by Lua.

To do

This PR is a Ready for Review.

How to test

Quoting Wuzzy2:

I recommend you to use luacmd and orienteering mods. Get yourself a speedometer with /giveme orienteering:speedometer and put it in the hotbar. This displays your speed at the top which is very useful for testing.

Now, in DevTest, call me:set_physics_override(table_of_physics_you_want_to_test). Test the normal walk speed and Fast Mode. Also test sneaking and climbing if you want to, just to make sure those still work as expected.
Then check if it does what you expect to / what the documentation promises.

Also test backwards compatibility with all different client/server combinations: this PR / 5.8.0 / pre-5.8.0 on the client + this PR / 5.8.0 / pre-5.8.0 on the server

[sfan5]

Code looks correct, feel free to merge if tested.