Possibility of passing an expression for the mappers config

**Is your feature request related to a problem? Please describe.**

Not related to a problem.
I'm using Mongoose and in order it to work properly I have to map the Mongoose Document interface of my models to the generated resolvers Types. The case it that the map list is getting quite big, once I have to manually add every new Mongoose model to the map list.

**Describe the solution you'd like**
A pattern that can be set for the mappers to resolve would be very handy.
Example:
```yaml
overwrite: true
schema: "./src/typeDefs/*.graphql"
documents: null
generates:
src/lib/generated/graphql.ts:
plugins:
- "typescript"
- "typescript-resolvers"
config:
contextType: ../graphql#Context
useIndexSignature: true
mapperPattern: ../mongoose/models/{{TypeName}}Model#{{TypeName}}Document

```
The above expression would resolve for each Type of the schema and produce a result like:
(Let's assume `TypeName` as equal to `User`)
`../mongoose/models/UserModel#UserDocument`

For the expressions that could not be resolved to a real reference, the default behavior can be assumed.
The maps explicitly set on `mappers` option, overrides the pattern one.

An additional option `mapperPatternExclude` as a list of Types names, can be used to exclude some Types to be parsed by the pattern expression.

**Describe alternatives you've considered**
A plugin that implements such behavior.

**Additional context**
Imagine a project with dozens or even hundreds of Types that have to be mapped.