feat: add getSummaryValues API (close #195, #240)

[dongwoo-kim]

Please check if the PR fulfills these requirements

• It's submitted to right branch according to our branching model
• It's right issue type on title
• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)
• The commit message follows our guidelines
• Tests for the changes have been added (for bug fixes/features)
• Docs have been added/updated (for bug fixes/features)
• It does not introduce a breaking change or has description for the breaking change

Description

Add getSummaryValues() method
(works only when the summary option exists)

Usage 1: with columnName

const summaryC1 = grid.getSummaryValues('c1');
console.log(summaryC1);
/*
{
  sum: 1000,
  avg: 200,
  max: 300,
  min: 50,
  cnt: 5
}
*/

Usage 2 : without columnName

const summaryAll = grid.getSummaryValues();
console.log(summaryAll);
/*
{
  c1: {
    sum: 1000,
    avg: 200,
    max: 300,
    min: 50,
    cnt: 5
  },
  c2: {
    sum: 2000,
    avg: 300,
    max: 600,
    min: 80,
    cnt: 5
  }
}
*/

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Thank you for your contribution to TOAST UI product. 🎉 😘 ✨

[sohee-lee7]

[1/29] 이견 없습니다. 고생하셨습니다 👍

[seonim-ryu]

[1/29] 저도 이견 없습니다. 👍