Implement log streaming command

**What**
- Add schema for log request and response
- Add command for printing the log stream
- Support passing since value as timestamp or duration
- Add some unit tests

Signed-off-by: Lucas Roesler <roesler.lucas@gmail.com>

schema/logs.go

@@ -0,0 +1,79 @@
+package schema
+
+import (
+	"fmt"
+	"net/url"
+	"strconv"
+	"time"
+)
+
+// import or alias these from the provider?
+
+// LogRequest is the query to return the function logs.
+type LogRequest struct {
+	// Name is the function name and is required
+	Name string `json:"name"`
+	// Instance is the optional container name, that allows you to request logs from a specific function instance
+	Instance string `json:"instance"`
+	// Since is the optional datetime value to start the logs from
+	Since *time.Time `json:"since"`
+	// Limit sets the maximum number of log messages to return, <=0 means unlimited
+	Limit int `json:"limit"`
+	// Follow is allows the user to request a stream of logs
+	Follow bool `json:"follow"`
+	// Pattern is an optional regexp value to filter the log messages
+	Pattern *string `json:"pattern"`
+	// Invert allows you to control if the Pattern should be matched or negated
+	Invert bool `json:"invert"`
+}
+
+// String implements that Stringer interface and prints the log Request in a consistent way that
+// allows you to safely compare if two requests have the same value.
+func (r LogRequest) String() string {
+	pattern := ""
+	if r.Pattern != nil {
+		pattern = *r.Pattern
+	}
+	return fmt.Sprintf("name:%s instance:%s since:%v limit:%d follow:%v pattern:%v invert:%v", r.Name, r.Instance, r.Since, r.Limit, r.Follow, pattern, r.Invert)
+}
+
+func (r *LogRequest) AsQueryValues() url.Values {
+	query := url.Values{}
+	query.Add("name", r.Name)
+	query.Add("follow", strconv.FormatBool(r.Follow))
+	if r.Instance != "" {
+		query.Add("instance", r.Instance)
+	}
+
+	if r.Since != nil {
+		query.Add("since", r.Since.Format(time.RFC3339))
+	}
+
+	if r.Limit != 0 {
+		query.Add("limit", strconv.Itoa(r.Limit))
+	}
+
+	if r.Pattern != nil {
+		query.Add("pattern", *r.Pattern)
+		query.Add("invert", strconv.FormatBool(r.Invert))
+	}
+
+	return query
+}
+
+// LogMessage is a specific log message from a function container log stream
+type LogMessage struct {
+	// Name is the function name
+	Name string `json:"name"`
+	// instance is the name/id of the specific function instance
+	Instance string `json:"instance"`
+	// Timestamp is the timestamp of when the log message was recorded
+	Timestamp time.Time `json:"timestamp"`
+	// Text is the raw log message content
+	Text string `json:"text"`
+}
+
+// String implements the Stringer interface and allows for nice and simple string formatting of a log Message.
+func (m LogMessage) String() string {
+	return fmt.Sprintf("%s %s (%s) %s", m.Timestamp.Format(time.RFC3339), m.Name, m.Instance, m.Text)
+}