cli/command/service: credentialSpecOpt: use strings.Cut

[thaJeztah]

• Rewrite the function to use strings.Cut instead of checking for, and trimming prefixes for each option.
• More explicitly set the value, instead of setting an empty value, then propagating the struct.
• Define a "type" to provide a more enum-like construct.

- Human readable description for the release notes

- A picture of a cute animal (not mandatory but encouraged)

[thaJeztah]

This was existing behavior, but felt a bit "odd" to set the value regardless if it was valid.

[thaJeztah]

Looking at go stdlib flags; the Set validates before setting the value (c.source is what's returned by c.String() for us); https://github.com/golang/go/blob/7a1679d7ae32dd8a01bd355413ee77ba517f5f43/src/flag/flag.go#L221-L233

func newUint64Value(val uint64, p *uint64) *uint64Value {
	*p = val
	return (*uint64Value)(p)
}

func (i *uint64Value) Set(s string) error {
	v, err := strconv.ParseUint(s, 0, 64)
	if err != nil {
		err = numError(err)
	}
	*i = uint64Value(v)
	return err
}

spf13/pflags also sets the value regardless (although the function used may return a default value);

cli/vendor/github.com/spf13/pflag/uint64.go

Lines 8 to 23 in 6686ada

	func newUint64Value(val uint64, p *uint64) *uint64Value {
	*p = val
	return (*uint64Value)(p)
	}
	func (i *uint64Value) Set(s string) error {
	v, err := strconv.ParseUint(s, 0, 64)
	*i = uint64Value(v)
	return err
	}
	func (i *uint64Value) Type() string {
	return "uint64"
	}
	func (i *uint64Value) String() string { return strconv.FormatUint(uint64(*i), 10) }

[codecov-commenter]

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✅ All modified and coverable lines are covered by tests.

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[thaJeztah]

This was also the previous behavior (where the value would be set to an empty struct regardless), but not sure if the value is used if an error is returned, so perhaps either skipping, or setting it to nil could work?