minor: add GetSchemaTypeMapping rest api

[amyXia1994]

1. Does this PR affect any open issues?(Y/N) and add issue references (e.g. "fix #123", "re #123".):

• N
• Y

2. What is the scope of this PR (e.g. component or file name):

pkg/service/rest_server.go

3. Provide a description of the PR(e.g. more details, effects, motivations or doc link):

minor: add GetSchemaTypeMapping rest api

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Other

4. Are there any breaking changes?(Y/N) and describe the breaking changes(e.g. more details, motivations or doc link):

• N
• Y

5. Are there test cases for these changes?(Y/N) select and add more details, references or doc links:

• Unit test
• Integration test
• Benchmark (add benchmark stats below)
• Manual test (add detailed scripts or steps below)
• Other

[coveralls]

Pull Request Test Coverage Report for Build 6185123685

• 1 of 6 (16.67%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.07%) to 64.605%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pkg/service/rest_server.go	1	6	16.67%

Totals
Change from base Build 6183142622:	-0.07%
Covered Lines:	2612
Relevant Lines:	4043

---

💛 - Coveralls

[Peefy]

LGTM