v4/signer: add codecommit git signing options

[taraspos]

Issue #, if available: aws/aws-sdk-go-v2#3310

Description of changes:
Adds following signer options:

• DisableUnsignedPayloadSentinel - allows to pass empty request payload without adding automatic UNSIGNED-PAYLOAD placeholder
• CustomTimeFormat - allows to customize time format used for request signing

Together, those options can be used to sign AWS CodeCommit Git HTTPS requests as described in aws/aws-sdk-go-v2#3310

Example usage

type CodeCommitCredentials struct {
	Username string
	Password string
}

type onlySignHost struct{}

func (r onlySignHost) IsSigned(header string) bool {
	return header == "host"
}

func CodeCommitGitCredentials(ctx context.Context, region, repository string) (CodeCommitCredentials, error) {
	cfg, err := config.LoadDefaultConfig(ctx, config.WithRegion(region))
	if err != nil {
		return CodeCommitCredentials{}, fmt.Errorf("failed to load AWS config: %w", err)
	}

	creds, err := cfg.Credentials.Retrieve(ctx)
	if err != nil {
		return CodeCommitCredentials{}, fmt.Errorf("failed to retrieve credentials: %w", err)
	}

	host := fmt.Sprintf("git-codecommit.%s.amazonaws.com", region)
	path := fmt.Sprintf("https://%s/v1/repos/%s", host, repository)

	req, err := http.NewRequest("GIT", path, nil)
	if err != nil {
		return CodeCommitCredentials{}, fmt.Errorf("failed to create request: %w", err)
	}

	sigTime := time.Now().UTC()

	signer := sigv4.New(func(o *sv4.SignerOptions) {
		o.HeaderRules = onlySignHost{}
		o.DisableUnsignedPayloadSentinel = true
		o.CustomTimeFormat = "20060102T150405"
	})
	signInput := &sigv4.SignRequestInput{
		Request: req,
		Service: "codecommit",
		Region:  region,
		Credentials: scredentials.Credentials{
			AccessKeyID:     creds.AccessKeyID,
			SecretAccessKey: creds.SecretAccessKey,
			SessionToken:    creds.SessionToken,
		},
		Time: sigTime,
	}
	err = signer.SignRequest(signInput)
	if err != nil {
		return CodeCommitCredentials{}, fmt.Errorf("failed to sign request: %w", err)
	}

	username := creds.AccessKeyID + "%" + creds.SessionToken

	authHeader := req.Header.Get("Authorization")
	sigStart := strings.Index(authHeader, "Signature=")
	signature := authHeader[sigStart+10:]
	sigBytes, _ := hex.DecodeString(signature)

	password := sigTime.Format("20060102T150405") + "Z" + hex.EncodeToString(sigBytes)

	return CodeCommitCredentials{
		Username: username,
		Password: password,
	}, nil
}

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.