Add direction to ListDocuments API

[chacha912]

What this PR does / why we need it:

Add direction to ListDocuments API to navigate to the next and previous pages.

Which issue(s) this PR fixes:

Address yorkie-team/dashboard#4

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

Additional documentation:

// If isForward is true, this request returns document lists with an id greater than previousID in ascending order.
// Otherwise, this request returns document lists with an id less than previousID in descending order.

listDocuments(previousID: string, pageSize: number, isForward: boolean)

Checklist:

• Added relevant tests or not required
• Didn't break anything

[CLAassistant]

All committers have signed the CLA.

[codecov]

Codecov Report

Merging #312 (430447b) into main (acdefd4) will increase coverage by 0.15%.
The diff coverage is 71.42%.

@@            Coverage Diff             @@
##             main     #312      +/-   ##
==========================================
+ Coverage   48.07%   48.23%   +0.15%     
==========================================
  Files          59       59              
  Lines        4903     4918      +15     
==========================================
+ Hits         2357     2372      +15     
  Misses       2285     2285              
  Partials      261      261              

Impacted Files	Coverage Δ
yorkie/backend/db/mongo/client.go	0.00% <0.00%> (ø)
yorkie/backend/db/memory/db.go	52.10% <100.00%> (+2.10%)	⬆️

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[hackerwins]

Thanks for your contribution. Overall it looks good.

I left some comments.

1. To improve the readability of the test code, I clean up as follows:

	t.Run("docInfo pagination test", func(t *testing.T) {
		localDB, err := memory.New()
		assert.NoError(t, err)

		assertKeys := func (expectedKeys []string, infos []*db.DocInfo) {
			var keys []string
			for _, info := range infos {
				keys = append(keys, info.CombinedKey)
			}
			assert.EqualValues(t, expectedKeys, keys)
		}

		pageSize := 5
		totalSize := 9
		clientInfo, _ := localDB.ActivateClient(ctx, t.Name())
		for i := 0; i < totalSize; i++ {
			_, err := localDB.FindDocInfoByKey(ctx, clientInfo, fmt.Sprintf("%d", i), true)
			assert.NoError(t, err)
		}

		// initial page, previousID is empty
		infos, err := localDB.FindDocInfosByPreviousIDAndPageSize(ctx, "", pageSize, false)
		assert.NoError(t, err)
		assertKeys([]string{"8","7","6","5","4"}, infos)

		// backward
		infos, err = localDB.FindDocInfosByPreviousIDAndPageSize(ctx, infos[len(infos)-1].ID, pageSize, false)
		assert.NoError(t, err)
		assertKeys([]string{"3","2","1","0"}, infos)

		// backward again
		emptyInfos, err := localDB.FindDocInfosByPreviousIDAndPageSize(ctx, infos[len(infos)-1].ID, pageSize, false)
		assert.NoError(t, err)
		assertKeys(nil, emptyInfos)

		// forward
		infos, err = localDB.FindDocInfosByPreviousIDAndPageSize(ctx, infos[0].ID, pageSize, true)
		assert.NoError(t, err)
		assertKeys([]string{"4","5","6","7","8"}, infos)

		// forward again
		emptyInfos, err = localDB.FindDocInfosByPreviousIDAndPageSize(ctx, infos[len(infos)-1].ID, pageSize, true)
		assert.NoError(t, err)
		assertKeys(nil, emptyInfos)
	}

2. Does MongoDB work in the same order(from the latest document) as MemoryDB?

[chacha912]

@hackerwins Thank you for your comment.

1. The code is much easier to read. Thanks👍
2. Yes. The FindDocInfosByPreviousIDAndPageSize function works the same way in MongoDB.