[csharp] make the method GetHttpSignedHeader of HttpSigningConfiguration public to get signed header.

[Ghufz]

HttpSigningConfiguration class provides the HttpSigned header which is used for httpSIgning Authentication. the function GetHttpSignedHeader of the class HttpSigningConfiguration was declared with access specifier internal which restrict user to get the signed header outside the assembly. to make the developer life easy and use the HttpSigningConfiguration class as utlity for httpSigning I have made the function to public so that it can be accessed outside assembly.

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• Run the following to build the project and update samples:

  ./mvnw clean package 
  ./bin/generate-samples.sh ./bin/configs/*.yaml
  ./bin/utils/export_docs_generators.sh
  

  (For Windows users, please run the script in Git BASH)
  Commit all changed files.
  This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master.
  These must match the expectations made by your contribution.
  You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*.
  IMPORTANT: Do NOT purge/delete any folders/files (e.g. tests) when regenerating the samples as manually written tests may be removed.
• File the PR against the correct branch: master (upcoming 7.1.0 minor release - breaking changes with fallbacks), 8.0.x (breaking changes without fallbacks)
• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.
  @wing328

[wing328]

appveyor (windows) tests passed via https://ci.appveyor.com/project/WilliamCheng/openapi-generator/builds/49687286

[wing328]

closed via #18496 (cherry-picking a commit in this PR) instead.

next time, please try to do a fresh fork of https://github.com/openAPITools/openapi-generator before making the change and filing the PR as that would help avoid 20+ unnecessary commits as illustrated in this PR.