Implement FormatType in Function::Format

[CEOJINSUNG]

Resolve #964

[coveralls]

Pull Request Test Coverage Report for Build 3350443590

• 76 of 89 (85.39%) changed or added relevant lines in 8 files are covered.
• 11 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.05%) to 97.838%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
core/src/executor/evaluate/mod.rs	2	4	50.0%
core/src/executor/evaluate/stateless.rs	2	4	50.0%
core/src/ast/function.rs	25	28	89.29%
core/src/executor/evaluate/function.rs	15	21	71.43%

Files with Coverage Reduction	New Missed Lines	%
core/src/executor/context/filter_context.rs	4	93.44%
core/src/executor/aggregate/mod.rs	7	81.82%

Totals
Change from base Build 3350243308:	-0.05%
Covered Lines:	38511
Relevant Lines:	39362

---

💛 - Coveralls