feat: use config file in billing server

[andy89923]

Description

• BillingServer uses config files setting (version 1.0.3)
• Update golangci-lint settings to comply with v1.57.2

[yccodr]

@andy89923 gomnd settings suggestion:

  gomnd:
    # !important in golangci-lint v1.58.0, gomnd is replaced by mnd
    # List of enabled checks, see https://github.com/tommy-muehle/go-mnd/#checks for description.
    # Default: ["argument", "case", "condition", "operation", "return", "assign"]
    checks:
      - argument
      - case
      - condition
      - operation
      - return
      - assign
    # List of numbers to exclude from analysis.
    # The numbers should be written as string.
    # Values always ignored: "1", "1.0", "0" and "0.0"
    # Default: []
    ignored-numbers:
    # List of file patterns to exclude from analysis.
    # Values always ignored: `.+_test.go`
    # Default: []
    ignored-files:
    # List of function patterns to exclude from analysis.
    # Following functions are always ignored: `time.Date`,
    # `strconv.FormatInt`, `strconv.FormatUint`, `strconv.FormatFloat`,
    # `strconv.ParseInt`, `strconv.ParseUint`, `strconv.ParseFloat`.
    # Default: []
    ignored-functions:
      - 'os\.Mkdir'
      - 'os\.MkdirAll'
      - '^math\.'
      - '^http\.StatusText$'