Closes #1628

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After I read the issue description and thought about it a bit, I figured there's conceptually three possible approaches to implement requested functionality:

1. By adding new attributes to @Transactional annotations like @Transactional(readPreference = PRIMARY)
2. By introducing new MongoDb-specific annotation for managing transactions
3. By re-using existing attributes of @Transactional

First option requires to add database-specific attributes to database-generic annotation. That doesn't sound right.

Second option can be implemented either by completely re-implementing/duplicating whole @Transactional functionality in Spring Data MongoDb or by altering @Transactional interceptor so that it can pass attributes of new annotation along with existing values. Even if that's possible, it will be too much effort. Also, splitting transaction configuration into two annotations kinda confusing.

So I implemented third option. The only suitable attribute in @Transactional to store MongoDb options is label. Spring docs suggest that:

Labels may serve a solely descriptive purpose or map to pre-defined transaction manager-specific options.

This PR adds possibility to add 4 labels that will be recognized as transaction options on per-@Transactional basis:

• mongo:maxCommitTime={value}
• mongo:readConcern={value}
• mongo:readPreference={value}
• mongo:writeConcern={value}

mongo:readConcern, mongo:readPreference, mongo:writeConcern values are converted from String to respective Mongo Client types using Mongo Client API methods.

mongo:maxCommitTime is converted from String using Duration.parse(String).

Example of usage:

  @Transactional(transactionManager = "txManager", label = { 
    "mongo:maxCommitTime=PT1M", 
    "mongo:readConcern=majority" ,
    "mongo:readPreference=primary" ,
    "mongo:writeConcern=acknowledged" ,
  })
  public Entity doSomeStuff(Entity entity) {
      //...
  }