refactor(provider): handling in config system

[appleboy]

• Update condition check to use String() method for openai.provider in commit.go
• Replace constants OPENAI and AZURE with Provider type and add String() and IsValid() methods in options.go
• Modify WithProvider function to use Provider type and validate with IsValid() method in options.go
• Change provider field in config struct from string to Provider type in options.go
• Update WithProvider calls in options_test.go to use String() method for Provider type

ref: #153