Support for overriding the GDB executable used for Zed's Debugger

[Altair-Bueno]

Why this is important

For debugging cross compiled code, as the normal GDB is only able to debug programs built for the host platform.

For debugging cross compiled code, one shall either use gdb-multiarch or some sort of $CROSSCOMPILE_PREFIX-gdb version provided by your SDK.

Current behavior

Zed scans the system for a binary called gdb and uses it to spawn the debugger. The relevant piece of code I believe is this one:

zed/crates/dap_adapters/src/gdb.rs

Line 169 in 15d59fc

.which(OsStr::new("gdb"))

Expected behavior

Zed invokes a previously set up executable as GDB. If unset, Zed searches the system for a binary called gdb and uses it.

Additional notes

It may be wise to add this to all other debuggers as a common option, as I'm sure lldbg users will have similar problems