Add /accesstoken endpoint

[t4niwa]

Description

Add /accesstoken endpoint

Type of change

• Bug fix
• New feature
• Refactoring (no functional changes, no api changes)
• Non-code changes (update documentation, pipeline, etc.)

Flags

• Breaks backward compatibility
• Requires a documentation update
• Has untestable code

Checklist

• Followed the guidelines in the CONTRIBUTING document
• Added prefix [skip ci]/[ci skip]/[no ci]/[skip actions]/[actions skip] in the PR title if necessary
• Tested and linted the code
• Commented the code
• Made corresponding changes to the documentation
• Passed all pipeline checking

Checklist for maintainer

• Use Squash and merge
• Double-confirm the merge message has prefix [skip ci]/[ci skip]/[no ci]/[skip actions]/[actions skip]
• Delete the branch after merge