feat(coderd/httpapi): add QueryParamParser.JSONStringMap (#16578)

This PR provides a convenience function for parsing a
`map[string]string` from a query parameter.

Context:
https://github.com/coder/coder/pull/16558#discussion_r1956190615

coderd/httpapi/queryparams.go

@@ -2,6 +2,7 @@ package httpapi
 
 import (
 	"database/sql"
+	"encoding/json"
 	"errors"
 	"fmt"
 	"net/url"
@@ -257,6 +258,23 @@ func (p *QueryParamParser) Strings(vals url.Values, def []string, queryParam str
 	})
 }
 
+func (p *QueryParamParser) JSONStringMap(vals url.Values, def map[string]string, queryParam string) map[string]string {
+	v, err := parseQueryParam(p, vals, func(v string) (map[string]string, error) {
+		var m map[string]string
+		if err := json.NewDecoder(strings.NewReader(v)).Decode(&m); err != nil {
+			return nil, err
+		}
+		return m, nil
+	}, def, queryParam)
+	if err != nil {
+		p.Errors = append(p.Errors, codersdk.ValidationError{
+			Field:  queryParam,
+			Detail: fmt.Sprintf("Query param %q must be a valid JSON object: %s", queryParam, err.Error()),
+		})
+	}
+	return v
+}
+
 // ValidEnum represents an enum that can be parsed and validated.
 type ValidEnum interface {
 	// Add more types as needed (avoid importing large dependency trees).