Support configuration variables on Gitea Actions (#24724)

Co-Author: @silverwind @wxiaoguang 
Replace: #24404

See:
- [defining configuration variables for multiple
workflows](https://docs.github.com/en/actions/learn-github-actions/variables#defining-configuration-variables-for-multiple-workflows)
- [vars
context](https://docs.github.com/en/actions/learn-github-actions/contexts#vars-context)

Related to:
- [x] protocol: https://gitea.com/gitea/actions-proto-def/pulls/7
- [x] act_runner: https://gitea.com/gitea/act_runner/pulls/157
- [x] act: https://gitea.com/gitea/act/pulls/43

#### Screenshoot
Create Variable:

![image](https://user-images.githubusercontent.com/33891828/236758288-032b7f64-44e7-48ea-b07d-de8b8b0e3729.png)


![image](https://user-images.githubusercontent.com/33891828/236758174-5203f64c-1d0e-4737-a5b0-62061dee86f8.png)

Workflow:
```yaml
  test_vars:
    runs-on: ubuntu-latest
    steps:
      - name: Print Custom Variables
        run: echo "${{ vars.test_key }}"
      - name: Try to print a non-exist var
        run: echo "${{ vars.NON_EXIST_VAR }}"
```

Actions Log:

![image](https://user-images.githubusercontent.com/33891828/236759075-af0c5950-368d-4758-a8ac-47a96e43b6e2.png)

---
This PR just implement the org / user (depends on the owner of the
current repository) and repo level variables, The Environment level
variables have not been implemented.
Because
[Environment](https://docs.github.com/en/actions/deployment/targeting-different-environments/using-environments-for-deployment#about-environments)
is a module separate from `Actions`. Maybe it would be better to create
a new PR to do it.

---------

Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: Giteabot <teabot@gitea.io>

models/actions/variable.go

@@ -0,0 +1,97 @@
+// Copyright 2023 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package actions
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"strings"
+
+	"code.gitea.io/gitea/models/db"
+	"code.gitea.io/gitea/modules/timeutil"
+	"code.gitea.io/gitea/modules/util"
+
+	"xorm.io/builder"
+)
+
+type ActionVariable struct {
+	ID          int64              `xorm:"pk autoincr"`
+	OwnerID     int64              `xorm:"UNIQUE(owner_repo_name)"`
+	RepoID      int64              `xorm:"INDEX UNIQUE(owner_repo_name)"`
+	Name        string             `xorm:"UNIQUE(owner_repo_name) NOT NULL"`
+	Data        string             `xorm:"LONGTEXT NOT NULL"`
+	CreatedUnix timeutil.TimeStamp `xorm:"created NOT NULL"`
+	UpdatedUnix timeutil.TimeStamp `xorm:"updated"`
+}
+
+func init() {
+	db.RegisterModel(new(ActionVariable))
+}
+
+func (v *ActionVariable) Validate() error {
+	if v.OwnerID == 0 && v.RepoID == 0 {
+		return errors.New("the variable is not bound to any scope")
+	}
+	return nil
+}
+
+func InsertVariable(ctx context.Context, ownerID, repoID int64, name, data string) (*ActionVariable, error) {
+	variable := &ActionVariable{
+		OwnerID: ownerID,
+		RepoID:  repoID,
+		Name:    strings.ToUpper(name),
+		Data:    data,
+	}
+	if err := variable.Validate(); err != nil {
+		return variable, err
+	}
+	return variable, db.Insert(ctx, variable)
+}
+
+type FindVariablesOpts struct {
+	db.ListOptions
+	OwnerID int64
+	RepoID  int64
+}
+
+func (opts *FindVariablesOpts) toConds() builder.Cond {
+	cond := builder.NewCond()
+	if opts.OwnerID > 0 {
+		cond = cond.And(builder.Eq{"owner_id": opts.OwnerID})
+	}
+	if opts.RepoID > 0 {
+		cond = cond.And(builder.Eq{"repo_id": opts.RepoID})
+	}
+	return cond
+}
+
+func FindVariables(ctx context.Context, opts FindVariablesOpts) ([]*ActionVariable, error) {
+	var variables []*ActionVariable
+	sess := db.GetEngine(ctx)
+	if opts.PageSize != 0 {
+		sess = db.SetSessionPagination(sess, &opts.ListOptions)
+	}
+	return variables, sess.Where(opts.toConds()).Find(&variables)
+}
+
+func GetVariableByID(ctx context.Context, variableID int64) (*ActionVariable, error) {
+	var variable ActionVariable
+	has, err := db.GetEngine(ctx).Where("id=?", variableID).Get(&variable)
+	if err != nil {
+		return nil, err
+	} else if !has {
+		return nil, fmt.Errorf("variable with id %d: %w", variableID, util.ErrNotExist)
+	}
+	return &variable, nil
+}
+
+func UpdateVariable(ctx context.Context, variable *ActionVariable) (bool, error) {
+	count, err := db.GetEngine(ctx).ID(variable.ID).Cols("name", "data").
+		Update(&ActionVariable{
+			Name: variable.Name,
+			Data: variable.Data,
+		})
+	return count != 0, err
+}