Add runCommand function to Go template syntax

This makes it possible to use date and time in initial values like this:

```yaml
initialValue: 'ruudk/{{ runCommand "date +\"%Y/%-m\"" }}/'
```

I want to use this to configure my BranchPrefix like this:

```yaml
git:
  branchPrefix: 'ruudk/{{ runCommand "date +\"%Y/%-m\"" }}/'
```

pkg/integration/tests/custom_commands/run_command.go

@@ -0,0 +1,42 @@
+package custom_commands
+
+import (
+	"github.com/jesseduffield/lazygit/pkg/config"
+	. "github.com/jesseduffield/lazygit/pkg/integration/components"
+)
+
+var RunCommand = NewIntegrationTest(NewIntegrationTestArgs{
+	Description:  "Using a custom command that uses runCommand template function in a prompt step",
+	ExtraCmdArgs: []string{},
+	Skip:         false,
+	SetupRepo: func(shell *Shell) {
+		shell.EmptyCommit("blah")
+	},
+	SetupConfig: func(cfg *config.AppConfig) {
+		cfg.GetUserConfig().CustomCommands = []config.CustomCommand{
+			{
+				Key:     "a",
+				Context: "localBranches",
+				Command: `git checkout {{.Form.Branch}}`,
+				Prompts: []config.CustomCommandPrompt{
+					{
+						Key:          "Branch",
+						Type:         "input",
+						Title:        "Enter a branch name",
+						InitialValue: "myprefix/{{ runCommand \"echo dynamic\" }}/",
+					},
+				},
+			},
+		}
+	},
+	Run: func(t *TestDriver, keys config.KeybindingConfig) {
+		t.Views().Branches().
+			Focus().
+			Press("a")
+
+		t.ExpectPopup().Prompt().
+			Title(Equals("Enter a branch name")).
+			InitialText(Contains("myprefix/dynamic/")).
+			Confirm()
+	},
+})