[add]mail support TLS port and add README.md for mail

[AboutSange]

Description

1. Add mail.SetTLS and mail.UnSetTLS functions
2. add README.md for mail

Motivation and Context

It solved: When using SMTP port 465 (TLS port) to send mail, always return error "failed to send email: EOF". And this problem is mentioned in https://github.com/nikoksr/notify/issues/269#issuecomment-1115986601

How Has This Been Tested?

1. run example code in README.md, and attempted to set smtpPort to 25, 465, 587 respectively, all send mail successfully
2. I write notify/service/mail/mail_send_test.go, and cd notify/service/mail/, and run go test, it's all PASS

package mail

import (
    "context"
    "crypto/tls"
    "log"
    "testing"

    "github.com/stretchr/testify/assert"
)

func TestMail_SendPort25(t *testing.T) {
    t.Parallel()

    m := New("xxx@163.com", "smtp.163.com:25")
    assert.False(t, m.useTLS)
    assert.Nil(t, m.tlsConfig)

    m.AuthenticateSMTP("", "xxx@163.com", "xxx", "smtp.163.com")
    m.AddReceivers("xxx@qq.com")

    // send a test message.
    err := m.Send(context.Background(),
        "SMTP port 25",
        "The actual message - Hello, you awesome gophers! :)",
    )
    if err != nil {
        log.Println(err)
    }
}

func TestMail_SendPort465(t *testing.T) {
    t.Parallel()

    m := New("xxx@163.com", "smtp.163.com:465")
    assert.False(t, m.useTLS)
    assert.Nil(t, m.tlsConfig)

    m.AuthenticateSMTP("", "xxx@163.com", "xxx", "smtp.163.com")
    m.AddReceivers("xxx@qq.com")
    m.SetTLS(&tls.Config{ServerName: "smtp.163.com"})
    assert.True(t, m.useTLS)
    assert.NotNil(t, m.tlsConfig)

    // send a test message.
    err := m.Send(context.Background(),
        "SMTP port 465",
        "The actual message - Hello, you awesome gophers! :)",
    )
    if err != nil {
        log.Println(err)
    }
}

func TestMail_SendPort587(t *testing.T) {
    t.Parallel()

    m := New("xxx@163.com", "smtp.163.com:587")
    assert.False(t, m.useTLS)
    assert.Nil(t, m.tlsConfig)

    m.AuthenticateSMTP("", "xxx@163.com", "xxx", "smtp.163.com")
    m.AddReceivers("xxx@qq.com")
    m.SetTLS(&tls.Config{ServerName: "smtp.163.com"})
    assert.True(t, m.useTLS)
    assert.NotNil(t, m.tlsConfig)

    m.UnSetTLS()
    assert.False(t, m.useTLS)

    m.SetTLS(&tls.Config{ServerName: "smtp.163.com"})
    assert.True(t, m.useTLS)
    assert.NotNil(t, m.tlsConfig)

    // send a test message.
    err := m.Send(context.Background(),
        "SMTP port 587",
        "The actual message - Hello, you awesome gophers! :)",
    )
    if err != nil {
        log.Println(err)
    }
}

Screenshots / Output (if appropriate):

go test result: all PASS

all mail was send success

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [ ] Documentation (no code change)
• [ ] Refactor (refactoring production code)
• [ ] Other

Checklist:

• [x] My code follows the code style of this project.
• [x] I have updated the documentation accordingly.
• [x] I have added tests to cover my changes.
• [x] All new and existing tests passed.