add strlen function

[zjbpaul1317]

What problem does this PR solve?

Briefly describe what this PR aims to solve. Include background context that will help reviewers understand the purpose of the PR.

Type of change

• New Feature (non-breaking change which adds functionality)

[JinHai-CN]

Since the result is filtered by strlen(c1) = 3, the output of strlen(c1) should be 3. Am I right?

SELECT *, strlen(c1) FROM test_varchar_filter WHERE strlen(c1) = 3;
----
abc abcd 5

[JinHai-CN]

Data type of varchar have two representation in Infinity. When check file src/parser/type/complex/varchar.h, you can find varchar data with length <= 13 and > 13 will have different memory layout. So the two cases should be tested in all function related with data type of varchar.

[zjbpaul1317]

Data type of varchar have two representation in Infinity. When check file src/parser/type/complex/varchar.h, you can find varchar data with length <= 13 and > 13 will have different memory layout. So the two cases should be tested in all function related with data type of varchar.

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