Add support for Delinea Secret Server

[pd-gov]

Add support for Delinea SecretSevers using their REST API.

This was tested against a private Instance. I don't know if there is a public demo app to test against.

[Copilot]

Pull request overview

This PR adds support for Delinea Secret Server as a new provider in vals, enabling users to fetch secrets from Delinea's REST API using the ref+secretserver:// URI scheme.

Key Changes:

• Implements new secretserver provider with support for retrieving individual secret fields and full secret maps
• Integrates provider into vals core runtime and string provider system
• Adds documentation for configuration and usage patterns

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 10 comments.

File	Description
vals.go	Registers the secretserver provider constant and factory in the runtime's provider preparation logic
pkg/stringprovider/stringprovider.go	Adds secretserver case to the string provider factory for provider instantiation
pkg/providers/secretserver/secretserver.go	Implements the new provider with HTTP client, authentication, and secret retrieval methods
README.md	Documents the secretserver provider with environment variables and usage examples

[pd-gov]

Applied the copilot changes and moved the URL from the ref url to an environment variable, to prevent exfiltration of the access token.

My code was essentially just a copy of the gitlab provider. Some of the Issues raised by copilot are present there as well.

[pd-gov]

fixed lint issue and rebased.