feat: support stamp substituions

[thesayyn]

Type of change

• New feature or functionality (change which adds functionality) [FR]: expand_template should support replacements, stamping #431

Test plan

• New test cases added

[thesayyn]

made sure that failures are isolated to missing toolchain hashes/artifacts for expand_template.

[gregmagolan]

Looks great. Lets split this up into two PRs for landing.

1. Bring in the toolchain and the golang binaries but don't change the existing expand_template rule

We'll then cut a patch release so the binary is published

2. Update the rule & reference the golang artifacts from the previous release

[gregmagolan]

Lets create a new expand_template.bzl load point for this rule with the PR and a new docs file for that so it shows up separately in the docsite. The expand_make_vars load point can stick around for now for backward compat with a # TODO(2.0): remove comment on it.

addressed valid concerns, and we have landed the go binary already.

[thesayyn]

Lets create a new expand_template.bzl load point for this rule with the PR and a new docs file for that so it shows up separately in the docsite. The expand_make_vars load point can stick around for now for backward compat with a # TODO(2.0): remove comment on it.

done

[gregmagolan]