Add crate versions in CHANGELOG (#2348)

* Move rendering external contributors to a function

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

* Add type alias `CrateVersionMetadataMap`

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>`.

* Render crate versions in CHANGELOG

This commit addresses a pain point brought up in https://github.com/awslabs/aws-sdk-rust/issues/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.

* Add a leading pipe when rendering markdown table

This commit addresses https://github.com/awslabs/smithy-rs/pull/2348#discussion_r1114164039



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Co-authored-by: Yuki Saito <awsaito@amazon.com>
Co-authored-by: Zelda Hessler <zhessler@amazon.com>