This commit moves the code for rendering external contributors to its own
function, in line with other render_* functions.

This commit adds a type alias `CrateVersionMetadataMap` for a field in
`VersionsManifest`. In a subsequent commit, the `changelogger` crate
will refer to this field, and it's better for that crate to not use the
field's bare type `BTreeMap<String, CrateVersion>`.

This commit addresses a pain point brought up in awslabs/aws-sdk-rust#731.
A binary `changelogger` now has a new command line option
`--current-release-versions-manifest` that points to the latest
`versions.toml` in the `aws-sdk-rust` repository. The program
will generate a markdown table showing crate versions from that
manifest and include it in an expand/collapse section.

This commit addresses #2348 (comment)

…github.com/awslabs/smithy-rs into ysaito/include-crate-versions-in-changelog