Remove access to the global gArgs for the fastprune argument and
replace it by adding a field to the existing BlockManager Options
struct.
When running `clang-tidy-diff` on this commit, there is a diagnostic
error: `unknown type name 'uint64_t' [clang-diagnostic-error] uint64_t
prune_target{0};`, which is fixed by including cstdint.
This should eventually allow users of the BlockManager to not rely on
the global gArgs and instead pass in their own options.
This is a partial backport of core#27125
https://github.com/bitcoin/bitcoin/pull/27125/commits/a498d699e3fdac5bfdb33020a1fd6c4a79989752
https://github.com/bitcoin/bitcoin/pull/27125/commits/18e5ba7c8002bcd473ee29ce4b5bfc56df6142a4
Depends on D16184