Adding the stub for the changelog and a config for generating it

This pr introduces git-cliff as a tool for generating (maintainable) change logs. The changelog attached to this pr was generated with this command:

git cliff -o -- "0bca116c1027a878469b72352e1e9e3916e85dde..0.1.0"

I added a place in the template to fill in "Human-generated change summaries" which I replaced with "First Release" message

To generate a new change log, you (or our internal tooling) will run

git cliff --prepend CHANGELOG.md --tag 0.2.0 -- "0.1.0.."

And you will get

An automated summary based on the new commits (These can be pruned as needed)
A place to add the human-written change summary
The rest of the change log

It will look like

# Changelog

All notable changes to this project will be documented in this file.

## [0.2.0](https://github.com/cloudflare/pingora/compare/0.1.0...0.2.0) - 2024-04-02
### Highlights
- Human-written change summaries go here

### 🚀 Features

- Enhance Server::new to accept impl Into<Option<T>> for ergonomics
- Implement Keep-Alive header parsing in Session

### 🐛 Bug Fixes

- Fix some comments

### ⚙️ Miscellaneous Tasks

- Ci: add github workflows (#3)
  Add CI workflows for docs, compile, test, and audit. Dependabot also
  included.
- We don't like dependabot (#86)
- Resolve TODOs in the pingora client example

### Everything Else

- Fix TinyUFO ahash in nightly Rust
  I think ahash has this bug where the hash of &u64 and u64 are different
  under nightly build. This works around that.
- Run upstream_response_filter on 304 from upstream cache revalidation
  Previously only the response_filter would run on revalidated responses
  with no way to process the upstream 304.
...

## [0.1.0] - 2024-02-28
### Highlights
- First Public Release of Pingora 🎉

cloudflare / pingora

Adding the stub for the changelog and a config for generating it #179