Issue 22: migrate to httr2

[AliSajid]

Pull Request

Description

This PR completely removes the dependency on httr, migrating to httr2 transparently. There are no major code changes or differences for the end-user.

Changes

• build: add httr2 as a dependency
• feat: add .ilincsBaseUrl to parametrize the base URL
• build: update the dependencies in renv
• feat: update the getSignature() method to use httr2
• feat: migrate getConcordants to httr2
• feat: remove references to httr

Related Issues

Closes #22

Also closes #77, #78 and #79

Checklist

• [x] I have tested these changes locally
• [x] I have updated relevant documentation
• [x] I have added/updated tests if applicable
• [x] All tests pass successfully
• [x] I have reviewed my code changes
• [x] I have addressed any feedback from code reviews