Allow overriding or omitting the revision metadata stamp #409

Since #383 (for #378), cargo-component always stamps the HEAD commit of the
package's git repository into the component's revision metadata section
(src/git.rs, and add_component_metadata in src/lib.rs, the
revision: git.map(...) field). There is no way to change or turn this off,
so identical sources built at different commits differ in bytes, and a build
outside git or from a source archive stamps nothing. That defeats
reproducible builds that compare or sign component bytes, and it makes the
stamp the HEAD commit even when the component's own sources did not change.

Today the only workaround is a post-build rewrite (wasm-tools strip --delete '^revision$' then wasm-tools metadata add --revision). That moves the
section to the end of the component, so its bytes differ from what
cargo-component would have produced with the same value.

Request: a setting to choose the revision, with a manifest form and an
environment form, the environment winning:

• [package.metadata.component] revision = "<literal>" stamps the literal,
  and revision = false omits the section.
• CARGO_COMPONENT_REVISION=<literal> stamps the literal, and an empty value
  omits the section. When set, it takes precedence over the manifest, so a
  build tool can stamp a source tree hash without editing the manifest.
• With neither set, behavior is unchanged: the HEAD commit.

Every case yields at most one revision section, in the position
cargo-component already uses, and a literal or a disable never consults git.

We carry this in a fork (https://github.com/attackordie/cargo-component/tree/feat/revision-setting) and would be glad to send it as a PR if the shape is
acceptable: about 115 added lines in src/metadata.rs (a RevisionSetting with a
custom deserializer that accepts a non-empty string or false) and
src/lib.rs (a resolve_revision step before add_component_metadata),
plus integration tests in tests/build.rs for the default, a manifest
literal, a manifest disable, a rejected revision = true, the environment
over the manifest, an empty environment value, and a re-stamp on a fresh
rebuild.

Searched existing issues and PRs for revision, revision section, git commit
hash, deterministic, source hash and CARGO_COMPONENT_REVISION: only #378
(closed, the request that added the stamp) and #383 (merged, its
implementation) came up.