useragent: add FromSlice function

[jar-b]

Description

This function accepts a slice of any value and converts to a slice of UserAgentProduct structs. This functionality will be used by upstream clients which accept a list of raw string content to be appended to the User-Agent header, rather than a structured object with name, version, and comment sections explicitly defined.

For example, this convenience method allows the existing Context function to be used as follows.

        var metadata []string
        // other processing steps
        ctx = useragent.Context(ctx, useragent.FromSlice(metadata))

Relates hashicorp/terraform-provider-aws#45464

[YakDriver]

LGTM 🎉

Couple minor nits

[YakDriver]

doesn't really need the empty check since fromString checks that?

[YakDriver]

Suggested change

	if len(subparts) == 2 { //nolint: mnd
	if len(subparts) == 2 && strings.HasSuffix(parts[1], ")") { //nolint: mnd

probably not worth "fixing" but an edge case "my-product/v1.2.3 (not a comment" that would parse wrong

[jar-b]

A missing closing parenthesis would still parse correctly because strings.TrimSuffix(subparts[1], ")") just does nothing. For example, this test case passes.

		{
			"malformed comment closing",
			"my-product/v1.2.3 (a comment",
			config.UserAgentProduct{
				Name:    "my-product",
				Version: "v1.2.3",
				Comment: "a comment",
			},
		},

--- PASS: Test_fromString (0.00s)

    --- PASS: Test_fromString/malformed_comment_closing (0.00s)

There is weirdness if the opening parenthesis is missed because the version and comment will get mashed together into the Version struct field, but it winds up rendering the same in the eventual UA.

In general this conversion logic is just a shim until V2 of the AWSCC provider, where we'll deprecate the structured object user_agent in favor of a list of strings. At that time the shared components in this repo can also be simplified to just pass the strings through as-is without all this extra manipulation.

[jar-b]

I'll still go ahead and add some test cases for malformed comment patterns, just so we document the edge cases exists and what the expected handling is.

[ewbankkit]

LGTM 🚀.