Improve /api/v1/pos/validator endpoint

[shurinov]

Right now there is no endpoint that returns information about a specific validator.

We have:
/api/v1/pos/validator: - summary: Get all validators, paginated

/api/v1/pos/validator/all: - summary: Get all validators, non paginated

I suggest adding a query for information about a specific validator. It could look like this:

/api/v1/pos/validator/{address}

or by adding query parameter address to existing endpoint /api/v1/pos/validator