Add new subcommands to changelogger for supporting file-per-change changelog

[ysaito1001]

Motivation and Context

Adds new subcommands --new and --ls to changelogger

Description

This is part 2 in a series of PRs supporting file-per-change changelog. This PR just adds utilities for humans and does NOT impact the current dev workflow, smithy-rs CI, or our release pipeline.

A subcommand --new

We can use this subcommand when creating a new changelog entry markdown file. An example usage:

$ changelogger new -t client -t aws-sdk-rust -r smithy-rs#1234 -a someone --bug-fix -m "Some changelog" \
# for long flags, replace -t with --applies-to, -r with --references, -a with --authors, and -m with --message \
# also remember to escape with \ when including backticks in the message at command line, e.g. \`pub\`

The following changelog entry has been written to "/Users/ysaito1001/src/smithy-rs/.changelog/5745197.md":
---
applies_to:
- aws-sdk-rust
- client
authors:
- someone
references:
- smithy-rs#1234
breaking: false
new_feature: false
bug_fix: true
---
Some changelog

The following CLI arguments are "logically" required

• --applies-to
• --ref
• --author
• --message

If any of the above is not passed a value at command line, then an editor is opened for further edit (which editor to open can be configured per the edit crate).

Note that the YAML syntax above is different from the single line array syntax proposed in the RFC. This is due to a limitation in serde-yaml (which has been archived unfortunately), but the multi-line values are still a valid YAML syntax and, most importantly, rendering changelong entries will continue working. For now, I won't post-process from the multi-line values syntax to the single line array syntax. One can work around this by handwriting a changelog entry Markdown file in smithy-rs/.changelog without using this subcommand.

A subcommand --ls

This subcommand will render a preview of changelog entries stored in smithy-rs/.changelog and print it to the standard output. Example usages (using the entry created at 5745197.md above):

$ changelogger ls -c smithy-rs \
# for long flag, replace -c with --change-set

Next changelog preview for smithy-rs
=====================================
**New this release:**
- :bug: (client, [smithy-rs#1234](https://github.com/smithy-lang/smithy-rs/issues/1234), @someone) Some changelog

**Contributors**
Thank you for your contributions! ❤
- @someone ([smithy-rs#1234](https://github.com/smithy-lang/smithy-rs/issues/1234))

$ changelogger ls -c aws-sdk \
# for long flag, replace -c with --change-set

Next changelog preview for aws-sdk-rust
========================================
**New this release:**
- :bug: ([smithy-rs#1234](https://github.com/smithy-lang/smithy-rs/issues/1234), @someone) Some changelog

**Contributors**
Thank you for your contributions! ❤
- @someone ([smithy-rs#1234](https://github.com/smithy-lang/smithy-rs/issues/1234))

Testing

• Existing tests in CI
• Basic unit tests for subcommands

---

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[github-actions]

A new generated diff is ready to view.

• No codegen difference in the AWS SDK
• No codegen difference in the Client Test
• No codegen difference in the Server Test
• No codegen difference in the Server Test Python
• No codegen difference in the Server Test Typescript

A new doc preview is ready to view.

[github-actions]

A new generated diff is ready to view.

• No codegen difference in the AWS SDK
• No codegen difference in the Client Test
• No codegen difference in the Server Test
• No codegen difference in the Server Test Python
• No codegen difference in the Server Test Typescript

A new doc preview is ready to view.