Add additional schema validation to poa module

[mitsuaki-u]

Expected behavior

Schema validation should be used when available

Actual behavior

In two locations in the proof of authority module, checks are done manually when schema
validation could be used instead.

endpoint.ts: the parameter given to getValidator is checked in the following way:

 const  {  address  }  =  context.params; 
 if  (  typeof  address  !==  'string'  )  { 
 throw  new  Error  (  'Parameter address must be a string.'  ); 
 } 
 cryptoAddress.validateLisk32Address(address); 

Check in getValidator
( lisk-sdk/framework/src/modules/poa/endpoint.ts#33-37 )

 validator.validate(getValidatorRequestSchema,  address) 

In commands/update_authority.ts the length of the command’s newValidators list is
checked in the following way:

 if  (newValidators.length  <  1  ||  newValidators.length  >  MAX_NUM_VALIDATORS)  { 
 throw  new  Error  ( 
 `newValidators length must be between 1 and  ${  MAX_NUM_VALIDATORS  } 
 (inclusive).`  , 
 ); 
 } 

Length check in updateAuthority
( lisk-sdk/framework/src/modules/poa/commands/update_authority.ts#33-37 )

Steps to reproduce

Check poa logic

Which version(s) does this affect? (Environment, OS, etc...)

v6.0.0