types: expose SchemaValidateFunction

[koba04]

What issue does this pull request resolve?

I'm trying to update ajv from v6 to v7.
In that way, we had to define the type of a validation function locally even ajv has the type definition.
This PR allows us to use SchemaValidateFunction to define a validation function.

• https://ajv.js.org/docs/keywords.html#define-keyword-with-validation-function
• https://github.com/kintone/js-sdk/pull/636/files#diff-d5a911011e45cad491e7becc159811e9b989c609694dc91a5aea51c797f17d40R14-R18

What changes did you make?

I've exposed SchemaValidateFunction from the entry point of ajv.

Is there anything that requires more attention while reviewing?

[koba04]

Thank you!