Allow custom TypeDescriptor field

[skadefro]

Is your feature request related to a problem? Please describe.
I want to use lidedb as a local cache for a system that uses mongodb as a backed.
But I already use _type for something else and that gives issues with litedb.

Describe the solution you'd like
I would like to suggest adding a TypeDescriptor property on the BsonMapper class to allow a custom name for this field

Describe alternatives you've considered
Have not considered any alternatives

Additional context
I have opened a pull request with the suggested feature.

[mbdavid]

Hi @skadefro,

If you are mapping documents between two databases that need this same _type column you need to consider create a custom BsonMapper class that use this new rules. You need to use a single object instance of this class in your LiteDatabase ctor.

[skadefro]

Hi mbdavid, I'm sorry if I'm missing something obvious, but I tried copying the BsonMapper class with my updates, to my own project, so I can use it in the LiteDatabase constructor.
But that will not work since LiteDB.Constants, LiteDB.Reflection, LiteDB.LinqExpressionVisitor and maybe more are all internal.