[csharp-netcore] Configure the API key in string format for HttpSigningConfiguration

[Ghufz]

Currently, the HttpSigningConfiguration object only accepts the API key in PEM/txt file format. However, if we need to retrieve the API key from a vault or another tool in string format, it is not possible to use it directly without saving it to a file.

To address this limitation, a pull request (PR) has been made to introduce a new property called KeyString. This property allows the user to provide the API key as a string. As a result, users now have the flexibility to either provide the API key file path using the existing parameter "KeyFilePath" or provide it as a string using the newly exposed property "KeyString".

Here is the example of API key as string.

var httpSignConfig = new HttpSigningConfiguration()
{
	KeyId = "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx",
	KeyString = "-----BEGIN RSA PRIVATE KEY-----\r\nxxxxxxxxxxxxxx\r\n-----END RSA PRIVATE KEY-----\r\n",
       HttpSigningHeader = new System.Collections.Generic.List<string> { "(request-target)", "Host", "Date", "Digest" }
}

Here is the sample code for API key file path

var httpSignConfig = new HttpSigningConfiguration()
{
	KeyId = "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx",
	KeyFilePath = "c:\users\xyz\apikey.txt",
       HttpSigningHeader = new System.Collections.Generic.List<string> { "(request-target)", "Host", "Date", "Digest" }
}

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/utils/export_docs_generators.sh
  

  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*.
  For Windows users, please run the script in Git BASH.
• File the PR against the correct branch: master (6.1.0) (minor release - breaking changes with fallbacks), 7.0.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.
  @vvb