[Fixes #197] Gracefully shutdown HTTP server

[dcarley]

To prevent a race condition seen in #197:

1. On the client:
   a. User runs process-compose down
   b. Client makes request to /project/stop
2. On the server:
   a. ProjectRunner.ShutDownProject() stops all processes
   b. cmd.runProject() stops blocking
   c. Exits before PcApi.ShutDownProject has written the response
3. On the client:
   a. Gets EOF reading the response because the server has gone away

I can't think of a way to reproduce this in a test, except for something
convoluted like creating a custom client that introduces a delay in
reading the response.

It can be "manually" simulated by adding a small delay here though:

diff --git a/src/api/pc_api.go b/src/api/pc_api.go
index d123257..d069399 100644
--- a/src/api/pc_api.go
+++ b/src/api/pc_api.go
@@ -3,6 +3,7 @@ package api
 import (
 	"net/http"
 	"strconv"
+	"time"
 
 	"github.com/f1bonacc1/process-compose/src/app"
 	"github.com/gin-gonic/gin"
@@ -268,6 +269,7 @@ func (api *PcApi) GetProcessPorts(c *gin.Context) {
 // @Router /project/stop [post]
 func (api *PcApi) ShutDownProject(c *gin.Context) {
 	api.project.ShutDownProject()
+	time.Sleep(10 * time.Millisecond)
 	c.JSON(http.StatusOK, gin.H{"status": "stopped"})
 }

And using this config:

processes:
  one:
    command: sleep infinity
  two:
    command: sleep infinity

Before this change:

bash-5.2$ go run src/main.go up --config services.yaml --tui=false &
[1] 8171
bash-5.2$ go run src/main.go down
24-07-12 15:38:40.332 FTL failed to stop project error="Post \"http://localhost:8080/project/stop\": EOF"
exit status 1
[1]+  Done                    go run src/main.go up --config services.yaml --tui=false

After this change:

bash-5.2$ go run src/main.go up --config services.yaml --tui=false &
[1] 8432
bash-5.2$ go run src/main.go down

[F1bonacc1]

That looks really good!
Thanks for fixing this so quickly!