Allow defining regular expressions for advanced error grouping

[jcreixell]

In some cases (for example finagle timeouts in the Scala client, when no stack trace is available) errors that have different messages are grouped together into a single exception type. This happens because the message is not used in the hashing function.

In other cases like, in the go library, in absence of exception types, the message is used for hashing, leading to too fine grained separation of exceptions.

It would be useful to allow defining regular expressions in the client for advanced grouping rules. These regular expressions could be propagated to the server in the exceptions endpoint to be reused for cross-instance aggregation, and potentially also for federation,