Restore the scripting general cache in preparation for deprecation of
the context cache, then removal of the context cache in master.

Brings back the general cache settings:
* `script.max_compilations_rate`
* `script.cache.expire`
* `script.cache.max_size`

`script.cache.max_size` and `script.cache.expire` are now dynamic settings.

The context cache is still default, the switch to general cache as default will happen on context cache deprecation.

System script contexts can now opt-out of compilation rate limiting using a flag rather than a sentinel rate limit value.  Duplicated defaults for system contexts have been coalesced.

Other than that, this code is the same as what was in 7.x to make a `master`-first commit and backport strategy easy.

Refs: elastic#62899
Backport: elastic#79414

Deprecate the script context cache in favor of the general cache.

Users should use the following settings:
`script.max_compilations_rate` to set the max compilation rate
  for user scripts such as filter scripts.  Certain script contexts
  that submit scripts outside of the control of the user are
  exempted from this rate limit.  Examples include runtime fields,
  ingest and watcher.

`script.cache.max_size` to set the max size of the cache.

`script.cache.expire` to set the expiration time for entries in
the cache.

Whats deprecated?
`script.max_compilations_rate: use-context`.  This special
setting value was used to turn on the script context-specific caches.

`script.context.$CONTEXT.cache_max_size`, use `script.cache.max_size`
instead.

`script.context.$CONTEXT.cache_expire`, use `script.cache.expire`
instead.

`script.context.$CONTEXT.max_compilations_rate`, use
`script.max_compilations_rate` instead.

The default cache size was increased from `100` to `3000`, which
was approximately the max cache size when using context-specific caches.

The default compilation rate limit was increased from `75/5m` to
`150/5m` to account for increasing uses of scripts.

Refs: elastic#62899