fix: Handling SIGTERM in CLI start command (sourcenetwork#1459)

## Relevant issue(s)

Resolves sourcenetwork#1458

## Description

Simply adds SIGTERM to the list of handled signals.

We can send send a signal,
`syscall.Kill(syscall.Getpid(), syscall.SIGINT)` within an integration
test but it will end the test process which also is running
the other tests.
It might require an 'external' integration test suite in which we can
run the command and then send the signal to the process.

## Tasks

- [x] I made sure the code is well commented, particularly
hard-to-understand areas.
- [x] I made sure the repository-held documentation is changed
accordingly.
- [x] I made sure the pull request title adheres to the conventional
commit style (the subset used in the project can be found in
[tools/configs/chglog/config.yml](tools/configs/chglog/config.yml)).
- [x] I made sure to discuss its limitations such as threats to
validity, vulnerability to mistake and misuse, robustness to
invalidation of assumptions, resource requirements, ...

## How has this been tested?

Manually

cli/start.go

@@ -18,6 +18,7 @@ import (
 	"os"
 	"os/signal"
 	"strings"
+	"syscall"
 
 	badger "github.com/dgraph-io/badger/v3"
 	grpc_middleware "github.com/grpc-ecosystem/go-grpc-middleware"
@@ -384,7 +385,7 @@ func start(ctx context.Context, cfg *config.Config) (*defraInstance, error) {
 func wait(ctx context.Context, di *defraInstance) error {
 	// setup signal handlers
 	signalCh := make(chan os.Signal, 1)
-	signal.Notify(signalCh, os.Interrupt)
+	signal.Notify(signalCh, os.Interrupt, syscall.SIGTERM)
 
 	select {
 	case <-ctx.Done():