Add usePullRequestMetadata option to use PR metadata instead of commit message

[knanao]

Overview

It enable to create release note to use PR's release block or PR's title instead of commit messages.

Issue

https://github.com/pipe-cd/actions-gh-release/issues/22

Note

Rate limit

This implementation call github REAT API in order to get PR metadata per merge-commit.
Hence, If one release contains a lot of merge-commits or you release really frequently, it’s possible to exceed the rate limit.
ref: Request from Github Action

When using GITHUB_TOKEN, the rate limit is 1,000 requests per hour per repository.
For requests to resources that belong to an enterprise account on GitHub.com,
GitHub Enterprise Cloud's rate limit applies, and the limit is 15,000 requests per hour per repository.

[knanao]

@nghialv
I fixed from your reviews.
PTAL, when you can do it.

[nghialv]

@knanao Great Job!

Just some nits. Please check them.

[nghialv]

Thank you so much. Here you go.
/approve