Implement mapBinding mssql dialect option

[kellyrbourg]

Purpose (Fix: Issue #5268)

Allow users of knex to define custom mssql parameter value/type mapping through configuration. This is useful when having a mix of varchar/nvarchar types within your database. Or if you are simply looking to extend the db type mapping that exists within knex.

For example, having all strings map to varchar.

knex({
    client: 'mssql',
    connection: {
      host,
      user,
      password,
      database,
      options: {
        mapBinding: value => {
          if (typeof value === 'string') {
            return {
              type: TYPES.VarChar,
              value
            }
          }
        }
      }
    }
  })

Changes

• added mapBinding type signature to MsSqlConnectionConfigBase
• added mapBinding config option check and call to mssql dialect.
• reworked _typeForBinding method to return the potentially mutated binding value to the calling code
• added integration tests for mapBinding

Documentation PR

• Documentation for PR#5292 documentation#440

[coveralls]

Coverage increased (+0.009%) to 92.28% when pulling 0a98d0f on kellyrbourg:feat/mssql-binding-mapping-config into 2dadde4 on knex:master.

[OlivierCavadenti]

Why only MsSQL dialect ?

[OlivierCavadenti]

can we remove this comment (not in your PR, but can be good to clean up)

[kellyrbourg]

Why only MsSQL dialect ?

Each dialect within knex has it's own sauce for how to go from nodejs types to database specific types. Some even defer to the external library for handling the mapping itself (AFAIK). Executing this PR for all dialects would require each knex dialect/client to own this mapping itself. Would need someone that knows how to do that mapping for the other dialects to help take that on. I'm not sure this is as big of an issue with other dialects either.

can we remove this comment (not in your PR, but can be good to clean up)

Will do.

[kellyrbourg]

Documentation PR created. knex/documentation#440