sentry-core: support custom user data in TransactionContext

[tommilligan]

Relates to recent work in #510

Adds support for the traces_sampler inspecting:

• first class properties of the TransactionContext
  • name
  • operation
  • sampled
• custom transaction context data, passed by the transaction starter

This is consistent with the Python implementation for custom sampling context.

Adds support for this custom payload by:

• making it gettable/settable in the same way sampled is; this is back compatible
• making it None by default, to minimise performance impact
• stored data is serde_json::Value:
  • a well established data type for arbitrary data in the Rust ecosystem
  • already a dependency in the sentry-core crate for internals of the v7 protocol
  • supports most data types useful to developers
  • mimics the existing behaviour of the python implementation (arbitary data, in a top level object with string keys)

[loewenheim]

Is set_custom_key meant to be the custom_insert method below?

[tommilligan]

Ah, yes. Will fix - I renamed it after I changed the signature to be in line with hashmap's insert (return previous value)

[Turbo87]

@tommilligan thanks for working on this!

would it make sense to extract the public getter functions to a dedicated PR to unblock them from getting merged and released? not sure how much discussion the custom user data feature needs... :)

[tommilligan]

@tommilligan thanks for working on this!

would it make sense to extract the public getter functions to a dedicated PR to unblock them from getting merged and released? not sure how much discussion the custom user data feature needs... :)

Fair point. Split the public getters into #514

[tommilligan]

Rebased on master to fix conflicts.