Add support for GOSSAFUNC/-ssafunc SSA debug flags in rules_go

## Description

**Background:** The Go compiler’s SSA backend can emit an interactive HTML dump of the SSA IR for a given function when you set the `GOSSAFUNC` environment variable:

```bash
GOSSAFUNC=MyFunc go build
```
This produces an ssa.html file showing all compilation phases, inlining decisions, and optimizations for MyFunc, which is invaluable when investigating performance regressions, validating correctness, or understanding the compiler’s IR transformations.

## Description

This produces an `ssa.html` file showing all compilation phases, inlining decisions, and optimizations for `MyFunc`, which is invaluable when investigating performance regressions, validating correctness, or understanding the compiler’s IR transformations.

## Problem

Currently, **rules\_go** offers no way to request SSA debug output from within a Bazel build:

* Passing `--action_env=GOSSAFUNC=…` has no effect, because Bazel’s sandbox strips most Go toolchain environment variables.
* There is no dedicated `ssafunc` attribute (or extension of `gc_goopts`) on `go_binary`, `go_library`, or `go_test`.
* Teams must build outside Bazel (via plain `go build`) to inspect SSA dumps, breaking hermeticity and reproducibility.

## Proposal

1. **Add a new `ssafunc` attribute** to `go_binary`, `go_library`, and `go_test`:

```starlark
go_binary(
name = "myapp",
srcs = ["main.go"],
ssafunc = ["main", "(*MyType).Compute"],
importpath = "github.com/example/myapp",
)
```

2. **Wire `ssafunc` into the compile action** by either:

* Setting `GOSSAFUNC=` in the sandboxed environment, or
* Passing `-ssafunc=` to `go tool compile` once available upstream.

3. **Emit the SSA HTML into the build output directory**:

```bash
bazel build //:myapp
open bazel-bin/myapp.ssa.html
```

4. **Document the feature** in the [rules\_go README](https://github.com/bazelbuild/rules_go#go-compile-options) alongside `gc_goopts` and `gc_linkopts`.

## Example

```starlark
load("@io_bazel_rules_go//go:def.bzl", "go_binary")

go_binary(
name = "server",
srcs = ["server.go"],
ssafunc = ["(*Server).handleRequest", "helperFunc"],
importpath = "github.com/example/server",
)
```

Building this target will produce `bazel-bin/server.ssa.html`, allowing developers to inspect SSA IR directly from a Bazel build.

Start with the go_binary, go_library, and go_test rule entry points and trace how their compile action handles gc_goopts and the sandboxed environment. Check the available Go compiler SSA flags and the rules_go README section on Go compile options. Done means a hermetic Bazel build accepts ssafunc, produces the requested SSA HTML in the stated output location, and documents the feature.

Written by the indexing model from the issue text.