Add exec config trimming to stabilize tool config hashes across modes

[rejuvenile]

Summary

• Adds a marker-based exec settings trim mechanism that resets target-specific Starlark settings in exec configuration
• rust_proc_macro transitions unconditionally reset settings (proc macros are always exec tools)
• Other rule transitions (rust_binary, rust_test, rust_static_library, rust_shared_library) propagate the reset when the marker is set
• Consolidates all rule transitions into a shared rule_transition_impl() function

Motivation

Bazel's built-in exec transition resets native flags (like --compilation_mode) but does NOT reset Starlark build settings. This means settings like --@rules_rust//rust/settings:lto=thin leak into the exec configuration hash, causing proc macros and build scripts to rebuild unnecessarily when switching between e.g. fastbuild and opt modes.

rules_rust already strips these settings at action construction time (via is_exec_configuration() checks), so the actual rustc commands for tools are identical. But the different config hash prevents cache reuse.

Settings reset in exec configuration

• Native flags: compilation_mode, stamp, strip, run_under
• Starlark settings: lto, codegen_units, extra_rustc_flags, extra_rustc_flag, extra_rustc_env, incremental, pipelined_compilation, no_std, experimental_per_crate_rustc_flag

How it works

1. rust_proc_macro's rule transition sets exec_settings_trimmed=True and resets all settings (via force=True)
2. When any rule's transition sees the marker is True, it also resets settings, propagating transitively through the exec dependency subgraph
3. The run_under flag is write-only (cannot be read in Starlark) so it's only included in proc macro outputs

Test plan

• Build with --compilation_mode=opt then --compilation_mode=fastbuild and verify proc macros are not rebuilt
• Verify --@rules_rust//rust/settings:lto=thin does not affect proc macro config hashes
• Verify normal target builds are unaffected (non-exec targets preserve all settings)