Sentrygin: Add TransactionNameGenerator option

[janwytze]

We would like to use c.Request.RequestURI in the transaction name instead of c.Request.URL.Path, but the transaction name is currently hard-coded. With the transactionNameGenerator a transaction name can be configured.

[cleptric]

For now, we'll move forward with #675 instead, where the new default is c.FullPath().

You can already overwrite the transaction name today, some options include using BeforeSendTransaction, receiving the transaction from the ctx transaction := sentry.TransactionFromContext(ctx) and setting its name transaction.Name = 'custom-name' or an event processor (called integrations).